Final Fantasy X Bosses

Strategy: Thwack twice and run away, run away! Cutscene interrupts after boss takes 90HP, so don't waste a potion.

Notes: I think Klikk's HP may reset to full when a new ally joins the battle.
Strategy: Hit and heal, hit and heal. Ally steals and tosses grenades (each time you steal, chance of another steal drops). If you want the overkill, delay until ally's overdrive builds, then Mix Grenades x2.

Special Attacks: Nautilus Charge whole party for ~150HP, Tentacles ~100HP
Prep: For overkill, fight extra pirhanas first to charge Rikku's overdrive a bit.
Strategy: Steal grenades and throw them when it goes to far side of room. Heal when HP drop below 200. After its second retreat, use Trigger Command "Pincer Attack". Cheer helps increase damage.

Notes: Endless Sinscales - 390HP, 200 overkill - means endless AP and up to 99 Power Spheres.
Strategy: Use Tidus' Cheer 3-4 times until you're killing Sinscales with one hit. Bash them until you get bored. Then use Kimahri's Lancet, Wakka's ball and Lulu's spells to hit Sin. When it's below 1000, charge Valefor's overdrive with Sinscales and then have her overkill Fin.
Tip: Heal Wakka and Tidus before finishing this battle.

Special Attacks/Notes: Every third turn, "Blender" does ~ 110HP damage to party.
Prep: When you first get on S.S. Liki, kick suitcase in hold for potions! (Not essential, just handy)
Strategy: Wakka's Dark Attack makes "Blender" miss and lasts 3 turns. Tidus' Cheer 5 times boosts defense and strength enough for a probable overkill; or wait to charge overdrives.

Special Attacks: Poisontouch, EARTHQUAKE to party when it wakes up.
Prep: Charge Valefor's overdrive with random battles, chat up Luzzu for Antidotes, and SAVE.
Strategy: Lulu's Fire does bulk of work. After Ochu falls asleep, it regens, so summon Valefor to hit it; she's immune to Earthquake. Then Dismiss her and bring her back in when he drops below 1200 for sure overdrive/overkill and rare spheres.
Notes: 40 AP (60 for overkill) means be sure to swap in everybody. You could wait until after you get Ifrit from Kilika Temple to use his Fire, but if you beat Ochu on the way, Luzzu and a few Crusaders in woods give extra items.

Notes: Geneaux starts out ARMORED, nearly impregnable to phys damage, and its two tentacles - 450HP (500 overkill!) - absorb magic.
Special Attacks: Sigh (Mag damage to party), Staccato (Phys dam to party), Venom (dam + Poison to target). Once shell is open, Water and Poison spells.
Strategy: Kill tentacles with physical damage, then swap in Lulu to crack shell. Use Wakka's Silence Attack if you've got it, or Yuna's NullTide. Summon Valefor at 1200 for overkill (She can take a few hits from it to charge up.)

Prep: Buy a few Echo Screens and Eyedrops from O'aka on the way.
Notes: Agility increases each turn. Crane will lower HP to 1/16.
Special Attacks: Blitzball Rush (party damage); often counterattacks with Dark or Silence.
Strategy: Haste Lulu. Have her zap the crane with Lightning 3x to jump start it. (Boy, wish the AAA agent looked like Lu.) Tidus operates crane for amusing/effective damage. Have Lulu Focus 4-5x so she can overkill, or else use her overdrive.

Prep: On Highroad to S, Lancet a Dual-Horn so Kimahri has Fire Breath ready.
Notes: **Armored while on feet; knock over or use Piercing weapons. It takes 1201 cumulative dam to knock him over, 501 more to push him back. Lvl 1 Key Spheres if he goes over cliff!
Special Attacks: Push Back whole party + damage; "You're Next" means Fist of Fury (1000HP, 1 target) next turn, Blizzard when on back.
Strategy: Start with Dark Attack. Haste Lulu for Fire, Auron for his Breaks. First time he falls over, Focus Lulu and Cheer to buff attacks. It's hard to push Eater off cliff before killing him: cheat by healing him! You can overkill with Ifrit, but I'm not sure you get the key spheres without pushing him off cliff.

Prep: Charge up aeons on way to Command Center and level Lulu to have at least 1 -ara spell.
Notes: Armored arms protect it; Lulu's -aras will lop them off with 1 spell each
Special Attacks: Body alternates between Punch (~600HP) and Demi; Head between Thunder and Venom (500+Poison)
Strategy: Haste Lulu to nail its arms, Auron to Power Break and pound it. Use Wakka's ball or spells to stop head. Use one aeon in each battle for overkill and lots of Key Spheres. In second battle, "ally" will take Lulu's place with -aras. (And no, his overdrive isn't worth waiting umpteen turns to see.)

Prep: How nice of O'aka to be selling Lightningstrike weapons on the way here!
Notes: Provoke cancels Depth Charge but increases Str for 2 rounds. 500 damage will also cancel it.
Special Attacks: 2 turns of Aqua Shooter for ~200; on turn 4 does Depth Charge, ~350 + Dark.
Strategy: Haste Tidus and Wakka, Slow the Extractor, and Cheer or try to get 2 lightning hits from Wakka's overdrive to overkill.

Prep: Switch to non-elemental weapons. Get Ixion's and Ifrit's overdrive charged. Save.
Notes: **Always vulnerable to one element, it absorbs/heals the rest. Each time it takes magic damage, it phases to a new element at random. Counters physical attacks with current element.
Special Attacks: Press does massive damage to party.
Strategy: Haste Yuna and Lulu, get your Nulls up and Steal. Have Lulu Focus about 4x to increase her power and party's defense. Now bring in someone to poke it with a stick (sword, etc), which causes it to give away its current element. Lulu counters with the opposite. Rinse and repeat.
For overkill and rare spheres, keep running tally of damage in your head, since you can't scan. When it gets down below 2000, use Lulu's -ara Fury or Ifrit's or Ixion's overdrive when it's the right element.

Prep: Charge Ixion's overdrive and hope you stole some lightning marbles in Thunder Plains!
Notes: Negator blocks summons and spells -- and Crawler's mana beam! Mana beam takes 4 turns to charge; Negator regenerates 3 turns after being hit. Do the math. :)
Special Attacks: Gatling Gun (damage party), Mana Beam (massive damage to 1), Assault counterattacks spellcasts
Strategy: Haste Rikku and Lulu, Slow the Crawler. Have Wakka knock down Negator. Rikku can use Lightning Gem even while Negator is up. Once it's down, Lulu Thundaras. (You might have Auron Guard her from Counterattacks). If it's about to unleash Mana Beam, bring in Ixion and Shield to soak damage, then have aeon cast Thundara on himself to heal. Ixion's overdrive, Lulu's Thundara Fury, or a Lightning Gem should overkill.

Notes: Guado Guards are 2000HP kill / overkill and self-heal until you overkill or steal from them.
Talking to Seymour raises Tidus' Str, Yuna's and Wakka's MDef.
After Seymour's down to 3000HP, summons Anima; his HP reset afterwards.
Attacks: S. multicasts ice, lightning, water, fire in that order; after Anima uses -ara spells which HURT.
Strategy: Talk. If available, use Kimahri's Stonebreath, Rikku's Petrify Grenades or Stonestrike weapons to remove guards. Meanwhile Haste Yuna and get up all four Nulls. Once she is ahead of Seymour with Null spells, Auron Magic Breaks and thwacks him until he calls for Mommy.

Mommy is Anima: 18,000 (1400) HP, Holy-weak, immune to all Breaks and Status Effects. The gang will remind you of your new aeon (scroll down in summons menu). Summon her to attack, heal her with Blizzara, or Shield when Anima unleashes overdrive. New aeon's overdrive kills Anima.

Now get back to Magic Break and pounding Seymour; the Nulls you cast earlier will save you from nasty multicasts. It won't take long to kill Seymour; one of your aeons can overkill.

Prep: Charge Ifrit while fleeing temple
Notes: Guado guards cast Berserk, Shell and Protect on him when killed, UNLESS they're petrified.
Special Attacks: Nasty Counterattack once he drops below 50% HP
Strategy: Remove Guado guards with Stonebreath/Petrify Grenades if possible, or steal their potions then kill them (2000HP; 1432overkill). Try Wakka's Sleep Attack and then tiptoe around beastie while Lulu burns him to a crisp. Auron's Threaten will cancel Berserk, and his Breaks are handy (but will wake Wendigo up). Haste helps, as usual, and Ifrit can deliver the overkill.

Prep: Equip Stone/Poison ward Armor, remove elemental weapons. SAVE.
Notes: Keep away 3 turns and Cid fires missiles. Also, no Poison/physical attacks while away. Evrae tends to cast Slow on Hasted characters.
Special Attacks: Breath Attack in melee does 1000HP to party + Poison; Photon attack at distance not so bad. Swooping Scythes to close in and smack party hard.
Strategy: This battle would be bad if Rikku didn't have Al Bhed potions curing poison and party damage. (Hopefully Kimahri too; I always start him on Rikku's path long enough to get Steal and Use). Start with Auron's Mental Break, then tell Cid to move off. Slow Evrae. Plink Evrae with spells, Wakka's ball, and gems so Cid can get off 3 rounds of missiles. Once Evrae refuses to stay away or Cid's out of ammo, Power Break Evrae and use someone's overdrive to finish Evrae off.

I'm not giving the full write-up because they're easy, but Grothia (Ifrit) is 8000 HP and ice-weak, Pterya (Valefor) is 12,000, and Spathi (Bahamut) is 20,000.

Forget Overkill; there's no AP bonus or drops.

Tip: as with any aeon duel, shield when the other aeon's about to overdrive; heal elemental aeons with their element. I use Bahamut vs. Grothia and Pterya, then Shiva dances with Spathi.

P.S. Did you get all the rare spheres in the Via Purifico? Here's a Via Purifico Map with directions on unlocking the teleport puzzle. Finding Lulu adds to Isaaru's cutscene.

P.P.S. This is where I plug my award-winning alternate universe sequel to FFX in which Yuna chooses the Final Summoning. Thirteen years later, Isaaru resumes his pilgrimage with Auron's grudging help.

Notes: This would be hard, if he weren't a ZOMBIE.
Special Attacks: Stone Gaze, shattering punch, Photon Spray on party
Strategy: Take 20 seconds to ponder: what hurts Zombies? Stumped? Okay, highlight the spoiler (which will spoil this battle forever): Toss 2 Phoenix Downs at it and collect your overkill.

Prep: Run around save sphere beforehand and charge Bahamut's overdrive in random battles.
Notes: When you KO Mortibody, it drains its new max HP from Seymour and regenerates. First it drops to 3000HP, then 2000, then sticks at 1000.
Special Attacks: Mortibody often uses Desperado, major damage to party + Dispel, if anyone's got multiple defensive spells up.
General Strategy: KO Mortibody so it uses its turn to suck HP from Seymour instead of hitting you. If you can put Reflect on Tidus, Provoke Seymour into bouncing his spells off Tidus.

THIS BATTLE HAS THREE PHASES:

1. Elemental Phase: Mortibody casts Fire/Blizzard/etc, wiping out the Null for that element, then Seymour multicasts the same element as -ara. Sometimes Mortibody hits party hard with Desperado doing damage + Dispel to cancel Haste, Reflect, etc.
   Strategy: Haste Yuna, get up Reflect if you have it or use Shell/Protect/Lunar Curtain. Have Lulu Bio Seymour so he's taking damage every turn. Talk to increase Strength or Mag Df. Steal from Seymour. Power Break and pound Mortibody as your primary target, Seymour with area spells/attacks/overdrives. Also cast Reflect on Seymour.
2. Petrify Phase: When Seymour drops below 24,000 HP, he Protects himself (or you if you Reflected him), then starts casting Break which petrifies 1 character.
   Strategy: Un-petrify with Al Bhed Potions and/or "kill" Mortibody before it uses Shattering Claw, which not only removes a petrified character from battle, but won't let anyone take his/her place! Keep pounding Mortibody as primary target, or do area attacks.
3. Flare Phase: When Seymour drops below 12,000, he starts casting Flare for 2000-2500HP damage on one target.
   Strategy: Reflect will save you. Mortibody casts Cura on Seymour or on party if you've put Reflect on Seymour. Bring in aeons with overdrives ready when he gets below 5,000 (or below 10,000 if you've got Bahamut ready).

Notes: Armored so only piercing weapons do much. Mighty Guard (Protect, Shell, Nulls) when critical.
Special Attacks: Blast Punch halves target's HP; Haymaker major damage to one target; SlowgaPrep: An aeon or two in overdrive. Aspirin and Ben-gay.
Strategy: Hastega, Dark Attack and Protect your party -- this thing's attacks are brutal. Haymaker usually misses if it's blind. If you're really in a jam, Provoke it and it'll just keep Blast Punching Tidus (how satisfying!) which simply halves his HP. Normally, I settle in with Yuna healing Lulu and Auron while they take turns smashing the thing. "Peel the fruit" for Lu with Mental Break. Once Yuna gets charged up for a Grand Summon, Dispel its Mighty Guard and bring in Bahamut.

Prep: Charge your aeons' overdrives with Magic Urns on the way in; have aeons plunk them for prizes until they blow up. (if explosion kills aeon, go to aeons menu and improve their Def/HP)
Special Attacks: Daigoro 1 target for 500; Kozuka 1 target for 1000 + Poison and MP drain; Wakizashi 1000 to everyone, and Zanmato = 9999 to party.
Notes: His overdrive Zanmato is almost certainly Game Over unless you bring in an aeon to shield.
Strategy: Hastega and hammer away. Lulu's -agas do plenty damage. If you get frustrated, Doom works, but aeons should trounce him.

Special Attacks: Bulldozer: hits Kimahri but separates them; Each Guards the other when side-by-side, reducing damage.
Notes: Lancet all 4 Ronso Rages from each one of them before killing either; they don't use Ronso Rage after their buddy is down.
Prep: Make sure Kimahri has STEAL ability. Optionally, improve chances of stealing by acquiring Master Thief auto-ability: see below. It's a prize in the Remiem Temple chocobo race.
Strategy: If Kimahri has Use, Chocobo Feather (Haste) will make your life easier. Then start Lanceting all their Ronso Rages: each has four, but Kimahri may already have some. Use Hi-Potions or Al Bhed Potions. Be sure to use an Overdrive EVERY time you Lancet (Mighty G and White Wind to heal are good ones). Finally, Steal many Level 3 Key Spheres from one of them. Each time you steal, chance of stealing goes down, but you can probably get up to five. Once you've stolen all you can from one Ronso, kill him, then Steal the other. Use area attacks like Aqua Breath or Grenades when they're together, or Fire Breath and single attacks when they're apart.

Special Attacks: (1) Seymour casts Zombie, Mort casts Full-Life to kill 1 target; (2) Seymour casts Reflect on self then Flare; (3) Mort casts Dispel, then Seymour Cross-Cleaves party for 2500 (Defend with Triangle button to halve this), (4) Seymour unleashes magical attack Total Annihilation which kills almost anyone without Shell.
Notes: Kimahri can raise Str with Talk command (not worth it); Yuna can raise her MDef (worth it). Every time you KO Mortichoris, it sucks HP from Seymour. (Its Max HP drop 1000 each KO until it reaches 1000). Seymour Banishes aeons on his turn, so...
Prep: Get all aeon overdrives charged; buy Holy Waters from Wantz. Equip any Silencestrike or Poisonstrike weapons you have; Seymour's vulnerable to those. If someone has Master Thief (see above), steal lots of Remedies from Grats on the way up Gagazet and customize 2-3 people's armor with Auto-med (takes 20). Then they'll cure Zombie status instantly.
Strategy: Poison Seymour, then keep attacking Mortichoris so it's sucking HP from Seymour instead of hitting you! For most of battle, I suggest Tidus to Haste and use Holy Water, Lulu for attacks and Auron because his Piercing weapons can damage Mort or Seymour. Yuna backing up.
Play-by-Play:
1. Hastega to start and cast Bio on Seymour. (Silence may also be useful.)
2. Seymour casts Lance of Atrophy to Zombie a character. Get Holy Water or Remedy on him/her before Mortichoris follows up with Full-Life, which kills a Zombie.
3. Steal, keep trying to Poison Seymour and/or use magic and Rikku's items to KO Mortichoris before....
4. Starting 3 turns in, Mortichoris often casts Dispel on party, then Seymour Cross-Cleaves whole party for ~2500 damage. If Mort casts Dispel, hit triangle so everyone defends against Cross-Cleave.
5. Seymour follows up casting Reflect on self + Flare, killing anyone not healed up. If someone gets a turn before he does, Dispel his Reflect and he'll Flare himself!
6. Also Dispel if Seymour casts Shell, Protect.
7. Keep bashing Mortichoris as primary target with Seymour as secondary (Poison + Mortichoris will be hurting him). Cut loose with aeons, but save Bahamut.
8. GET READY when Seymour starts charging: you'll need everyone with Shell up or else bring in an aeon to block TOTAL ANNIHILATION attack which tends to kill party.
9. Once Seymour's down to 15,000 or so, bring in Bahamut with overdrive to get the overkill.

Notes: This battle would be hard, if not for a secret: this beast is SCARED OF AURON. When Keeper gets below 50% HP, it casts Curaga on self -- or on party, if you put Reflect on it!
Special Attacks: Mana Breath usually kills target (esp. an aeon); Tail Sweep damages party + Delay; Photon Wings damages party + Dark, Poison, Curse (blocks overdrives; cured by Holy Water)
Prep: An aeon or two in overdrive for insurance, Holy Water, and equip any Curseproof, Darkproof, Poisonproof armor you've got.
Strategy: Hastega, Bio the beastie and let Auron rip with Power/ Armor/ Mental Breaks or Banishing Blade while Yuna keeps him alive (sort of). Be aware: Keeper is SMART, and will often counter these with Protect and/or Shell (but this wastes its turn). Likewise, don't cast Slow, or it'll cast Haste. It may Dispel if you cast Reflect on it, but worth it if you can get it to bounce Curaga on you! Aeon overdrives can wittle it down, but it mashes them like bugs.
If you're really in trouble, stop, take a deep breath, and scan through Auron's special abilities. There's one we never use that will turn this thing into a bunny rabbit. (Highlight if stumped: Threaten) It makes this battle far too easy, but proves again that Auron is Made of Awesome.

Notes: You'll need aeons charged for next battle, so use them sparingly; however, Valefor dominates this guy because mines can't reach her and she dodges well.
Special Attacks: Sets "Glyph Mines" on platforms at random that blow up a turn later. Berserk Tail causes Berserk (duh). Counterattacks by slashing 3 characters in front of it.
Prep: I recommend charging overdrives for Valefor and Ifrit, but it's not essential.
Strategy: Hastega really helps. Mental Break and hit it with Flare (it's immune to Ultima/Demi). Take your time: keep moving characters behind it and out of the way of mines. Don't let more than one person get Berserked; Esuna/Remedy cures it. Berserk on Auron, Wakka, or Tidus is okay: it makes them stronger. Just Life and Re-Haste them when they go down. You'll probably need an aeon overdrive to kill it. Try Valefor.

Special Attacks: See Strategy below
Prep: If Yuna's close to Holy on Sphere Grid, go back to Dome and level her up for Holy. Charge aeon overdrives. Equip any Confuseproof armor you have. Silenceproof on mages also a good idea. Holy Water is useful but not essential (steal it from the Y-machinas in Zanarkand). Teach aeons Dispel if you can.

Form 1: Really Big Hair: Warm-up. Don't use aeons. Hastega everyone, Steal, Cheer and use Reflex/Focus to boost defenses. It's not hard to kill this form. She often counters phys attacks with Blind, magic with Silence, and skills with Sleep, so have Esunas, Remedies, or appropriate item ready.

Form 2: Snake Monster: It takes a while to whittle her down -- quicker if Yuna's got Holy -- but save overdrives for last form. Yunalesca starts using Hellbiter on whole party to Zombie them (200 HP damage each). Sometimes she casts Cura or Regen on Zombied characters, hurting them. But wait! Zombie is useful later, so leave tougher characters like Auron in Zombie mode. If Yunalesca casts Regen on them, Dispel it. Have Zombies ready with HP near full before killing this form.
Alternatively, remove Zombie status with Holy Water and watch Yunalesca's HP carefully to summon an aeon to knock off form 2.

Form 3: Medusa EWW: First thing she does is cast Megadeath which will kill anyone who's not a Zombie. Aeons are also immune to Megadeath. Then she casts Curaga, so if there's only one Zombie standing, that will KO him/her. Yunalesca will continue to Hellbiter/Megadeath fairly often, so try and keep a balance of new Zombies who are still at high HP, and weak ones whom you can save with Holy Water. Or just let them KO and resurrect them. Even if you're using aeons, you need a few Zombies in case she kills an aeon and then casts Megadeath.
Her Mind Blast also Confuses + Curses people, stopping their overdrives.
Once you get Form 3, THEN unleash every overdrive and aeon in your arsenal. Yunalesca Curses aeons which disables their overdrive, plus she tends to Osmose HP and MP from them. If you've taught aeons Dispel, they can remove the curse.

Bring in Bahamut for overkill; he's the only thing that can do it unless you've gotten Yuna's or Kimahri's Celestial Weapon to power up Valefor or Ixion.

Notes: 3 boss battles in a row before you rest.
Every fourth turn, fin rests to gather energy; next turn is Gravija.
Special Attacks: Gravija reduces party HP by 1/4; no effect at a distance. Negation, which is a speedy counterattack, removes bad statuses on Fin. ALSO removes good status on you if near.
Prep: Consider powering up 1-2 Celestial Weapons to Break Damage Limit for a couple characters and aeons, to make overkill easier.
Strategy: Hastega, Mental Break, Armor Break, Steal, then pull away. Pound with strongest magic, distance attacks or aeons' special attacks (Heavenly Strike, Impulse, etc). If Rikku has Copycat, copy Lulu's spells or Holy. You can fight close range if that's too slow, but be ready to heal after Gravija before next attack starts dropping people. Replace Hastega after a Negation at close range.

Prep: Get Firestrike weapons and/or Poisonproof armor; remove Waterstrike.
Notes: Sinspawn Genais is in front of Core, keeping party at distance from Core until killed. Genais absorbs magic attacks aimed at Core, heals with Cura, and counters magic attacks with Waterga. When Genais loses half HP, retreats into shell, then armored and immune to Ultima. Core counters attacks on itself (including magic) with low-level elemental spell on all 3 party members.
Special Attacks: Genais mostly poison-based attacks. Inside its shell, uses Sigh, another poison attack, while Core uses Gravija, or may do Toxic Cloud after Gravija which can be deadly.
Strategy: Hastega, Slow Genais so its turn and Core don't have turns back-to-back. Firestrike Genais. Use Silence Buster and Zombie Attack on it if you can; it's resistant to both, but if you get through, that stops Cura. Once Genais is dead, Armor Break core and have at it. "Gathers Energy" means Core will Gravija next turn; bring in an aeon and use Shield to soak it.

Notes: You must finish battle before Sin's overdrive gauge is full or you DIE.
Special Attacks: Petrifiy/Confuse/Zombie Gaze; after about 16 turns Giga-Graviton overdrive = GAME OVER.
Prep: Break out Stoneproof, Confuseproof, Zombieproof armor.
Strategy: Hastega as usual. Save overdrives/aeons for finishing blow; they waste too much time (again, a quick Scan at the start will let you know when it's within 10K overkill range). Lulu's Doublecast and Rikku's Copycat help a lot, as do Flare, Ultima, Holy. After 2-3 "Sin draws you in" messages, you'll be in hand-to-hand range; Mental Break and Armor Break.

Prep: You may want Shell or [Element] Eater armor.
Special Abilities:Multicasts element spells 4x per turn, -aga or -ara. Occasionally casts Dispel, then Ultima next turn.
Notes: Seymour has "Mortiphasm" wheels behind him. Red = Fire, Dark blue = Ice, Light blue = Water, Yellow = Thunder. Here's how they work:

• All 4 same color = multicast -aga of that element, FOUR TIMES.
  
• 3 of same color = multicast -aga of that element 3 times, then -ara of the single element.
  
• Anything else, he casts -aras for each element.
  
• 3x of same color = he absorbs that element; 1-2x, he takes half damage.
  
• Attack a Mortiphasm Wheel with a spell to turn it right. Physical attacks turn it left.

Strategy: Hastega, get up Reflect on your party and/or Nulls, bop two of the wheels with low-level spells or Wakka's ball to scramble them. Mental Break and Power Break Seymour, and if all his colors are about to match, bop them again. Or just prep with elemental proof or -eater armor, use Shell and Reflect to soak the damage, and pound him.
Important: Whenever he casts Dispel, gloats, and starts glowing red, he's about to Ultima your ass. Bring in an aeon with Shield to block it.

Notes: "Yu Pagodas" floating beside Jecht heal him 1500 points each and may remove status effects. They may be stopped for a few turns with damage (5000K each). Tidus can use Trigger command "Talk" to reset Jecht's overdrive gauge to zero. Works only 2 times, so save it for Jecht's second & final form.
Special Attacks: Jecht Beam causes petrification and character may then be shattered by physical attack; regular sword-swipe hits whole party; Triumphant Grasp hurts and Zombies one character; Jecht Bomber does 4000K to aeon; 2nd form's Ultimate Jecht Shot hits everybody for ~5000K.
Prep: Again, Break Damage Limit for aeons helps. Equip Zombieproof, Stoneproof armor; also Zombiestrike and/or Poisonstrike weapons. Check Rikku's Mixes and see if you've got ingredients for any protect-related buffs like Mighty G.
Strategy: First battle, Hastega, Scan, then use aeons' overdrives. Jecht will clobber them but oh well. Make SURE you remove petrification on any character as they can be Shattered next turn. Between Yuna's turns, get Rikku or others to use Light Curtains or Mixes to Protect party members.
Second half of battle, Hastega, Zombie Strike, Mental Break, then use Ultima to clobber Jecht + Yu Pagodas. Power Break and Armor Break work too, but I think Power Wave removes status effects. Zombie will wear off, but as long as it's on, Yu Pagodas will hurt instead of healing Jecht. TALK TO JECHT with Tidus' trigger command each time you see the "Jecht Overdrive" warning. Use up aeons/overdrives and powerful items you were saving. Doublecast Flare/Ultima/Holy or use Quick Hit.

Final Notes: I'm not going to give you a strategy on this one for reasons that will quickly become clear. I will say only this:

if some of your aeons survived Jecht, you might as well do aeon vs. aeon; and remember what you learned about Yu Pagodas in the battle with Jecht. It's still true.

When you're done, pat yourself on the back -- congratulations! -- then go watch this incredibly cool Fan Tribute Video for Auron that gives him the send-off he deserves.