Orange Box Resource

Conservationist: This one came really easily for me, but just take one of your pulse rifles energy balls and chuck it into a room with quite a few combine and then promptly stare at the bottom of your screen, waiting to be rewarded.

Blast from the Past: As soon as you enter the junkyard, it's to your left, way up high, on top of some levels of storage containers and cement. I think it resides in a container too (just a see through thing for holding stuff, it won't obscure your vision)

Zombie Chopper: This one is very easy, aside from a few parts at the very end. Just remember the health cheat if you find yourself staring at a red screen a lot. Look for any items that explode, cut, or weigh more than 20 pounds. These kill quite efficiently.

Keep Off the Sand!: Just jump a lot, and use the objects littering the ground to support yourself if you have to cross large stretches of sand. This one will force you to take quite a long while at the task, so be patient.

Flushed: I believe there may be a loose toilet or two in nova prospect, but I missed the achievement there, so I grabbed it during the battles in city 17, unfortunately I can't quite remember where. I went through checking every single bathroom, but that's a waste of time because they're all bolted to the floor. What you want to look for is a particularly crappy bathroom, or a junk pile that might contain that sort of thing. Then just pick it up and toss it at some poor hapless combine soldier. Just make sure you do it quickly and accurately, because the toilets tend to fall apart rather easily.

Atomizer: The field you need to throw the combine into are the circular shafts that slow down objects, you know, the ones with really cool physics effects. Yeah, grab some combine suckers with your super gravity gun and shove them in there. Be careful, try to get as many in as you can, because there are only something like three of these fields.

Lambda Locator: This one is quite a challenge. I posted videos, not by me, that guide you excellently to each location, although the music is at times unbearable and he takes a while to get to the actual location. Just keep your eyes open and try to remember which ones you have and have not gotten when you watch the videos. If you see something out of the ordinary, go check it out to make sure you already have it.

HotPotatOwned: This one may not have proven so difficult for others, I don't know, but it was extremely hard for me to finally acquire, but boy was I happy when I did. I finally decided it was now or never when I came upon the part where you face off against the Antlion guardian. Right after that some combine start filling in around their wall. What I did was I went up against the building to the right of the wall, looking at it from Alyx when she's on the machine gun, and waited till the combine started filing out of the wall when it's doors opened. The guys around there liked to throw grenades a lot so I decided it was a pretty good time to grab the achievement. I picked up a nade thrown at me from the doors, then tossed it right back and it landed at the poor bastard's feet with only half a second left to spare. Oh, well.

Zombie-Que: This one was a bit hard, but not that much. Basically, when you're underground, around the dark areas, look for flares and then just pick them up with your hands or the gravity gun and walk around sticking them in zombie faces. Your last chance IN THIS PART OF THE GAME is right before you get on the elevator with Alyx, and there are infinite zombies at the end, so go all out with the flares down there, and don't get too angry when they keep swatting the flares from your grasp. If you don't get it at this part, much later, right after the water filled room with electric cables and barnacles, where you get separated from Alyx, you come to a room where there is fire, a headcrab canister, and rubble. There will also be a poison zombie and some doors that zombies will start busting through. In this room there are two flares, so if you didn't get the achievement earlier, then this is your last chance, but it's a good chance, too.

Pacifist: The idea on this one is to get through the citadel core part without harming any stalkers. This pertains only to stalkers in the room, so the stalkers earlier on in the level are alright to kill, maim, and destroy. What you'll want to do is quickly grab one energy ball after another and fill the outlets on the walls with them and sprint quickly across the bridges before the stalkers can contradict your actions. They do take a while, but they start burning the balls out of the outlets almost as soon as you get them in there, so the trick to this one is speed.

Live Bait: This one involves Alyx sniping zombies and antlions and combine during ONE particular section, in other words, there are no other chances at this one. Basically, have her kill all of the zombies up until the street with the combine on the raised platform and the antlions. Here you'll want to just camp in a corner and knock the antlions over so Alyx can snipe them. Since the antlions are infinite until you plug their holes, that makes this part overly simplistic. Just thought my advice would be helpful.

The one free bullet: I'd already played the game twice on PC when I decided to get it for the 360 and relive the glory days, so this one was probably much easier for me, but I still did it on my first run through. Basically, you can use the ammo cheat (Y, B, A, X, Right Bumper, Y, X, A, B, Right Bumper) to give yourself a lot of rocket ammo and use that when you need to shoot at something, and the gravity gun, when holding anything heavy, becomes the ultimate weapon, and grenades help, too. Crowbar for zombies and headcrabs. This achievement seems overwhelming and ridiculously hard. Only some parts are magnified in difficulty by trying to achieve this. It actually doesn't make the game that much harder. You don't NEED any cheats to accomplish it, but use the ammo one if you want and the health cheat (Up, Up, Down, Down, Left, Right, Left, Right, B, A) helps out quite a bit as well.

Little Rocket Man: Alright, the real pain in the butt. He's in the communications room at the beginning of the game where you first see Dr. Magnuson and Alyx talks with her father and Kleiner. It's in the back corner of the room, just rummage around a bit and you'll find it. Now, yes, I'm going to be blunt and honest, you are going to have to take the thing everywhere with you. BUT, you get some leeway here and there. When in combat, or when you are near something that may disrupt your courier mission, drop it in a safe place that you will easily remember and come back for it later. Also, when you are defending the underground outpost, just drop the gnome by Alyx's table and make sure to grab it when you come back up to collect her. Then, drop it next to the sliding door when you reach the part where Alyx snipes for you, you eventually open that door for them and can come back in and get it. From then on, you have to cart it around in the car with you. Just put it in the door of the rocket when you finally get there, shut the door and voila! ACHIEVEMENT UNLOCKED!

Get Some Grub: This guide will help you more than anything else: GAMEFAQS
The basic idea of this one is to look everywhere, floor, ceiling, walls, tunnels, web caches, and in the most obscure places you can think of. There won't be any underwater so don't worry about that. Make sure that you get the 2 hidden ones, one being in a locker near the part where you encounter multiple acid lions at once, and the other being at the end of the underground section, after you have Alyx back, when the guardian disturbs you and she comments on how you stole its eggs, located back where the guardian was, on the wall.

Pinata Party: As long as you keep your eyes and ears posted for this one, you should be fine. It's pretty simple, just look for tiny tunnels that have their entrances covered in webs, then shoot the webs and voila! Make sure to get all the caches in the cavernous room when you power up the elevator and bring the vortigaunt down.

Puttin' on a Clinic: The trick to this one is saving right after you have successfully scored a hit on the chopper, that way, if you miss, you can just quickly reload your game and continue bashing the thing to death with its own bombs. It's good practice for the eventual ending of the game, where you'll have to accomplish a similar task.

Cache checker: Just follow your radar, it helps a lot. The hardest one is the house, where you'll want to shoot out the floor, pretty much the whole floor, and there's a ton of supplies down there, too.

Pedal to the Metal: Just turbo a lot, and make sure to get ahead of dog early on, take your corners smoothly, confidently, and you should have this one made.

Neighborhood Watch: This one is somewhat difficult, but it becomes a lot easier with practice and with what you're about to read. For the first one, let it come to you, then turn around, jump in your car, and drive straight to the second, and drop it before it gets the chance to fire. From then on out, just try to meet every strider as close to its spawn point as possible. Run the hunters over and try to take the striders in chronological order (first to last). That should generally set you on the right track.

Payback: I grabbed this one when the hunters ambushed me at the radar station with Alyx. Just pick up an object (I used a tire) and get some of the hunter's flechettes on it, then punt the object back at the hunter. If you're having trouble getting this one, try waiting a second before punting it back.