Metro 2033

This is what I did.

Go to the loading a mission screen and find Frontlines. If you remember that the mission before Frontlines has you in Armory. You should load Armory and go through the capture bits and then before you jump into the pit go to the dealers. Buy a crap load of ball bearing ammo and then jump into the pit. (unless of course you already have a bunch of ball bearing ammo ) better to be safe as this gun eats ammo and well you know how some soldiers just stand and take a full round to the face and then run off.

When you reach Frontlines just run past the cart in-front of you and keep going forward until you see an old wrecked train and hop in and at the back you will find the Pneumatic gun. ( Hold right bumper and press the right trigger to pump it up ) You will probably have enemies shooting at you so face out the train and stay on the right and kneel down and pop off those enemies. Then proceed through the level popping enemies. Keep pumping the gun as it takes them down much quicker and don't forget those two wounded soldiers.

You will get the achievement before even entering the red colored tunnel.

threre are a couple places you can find pneumatic guns early on in the game.

1. the first one you can find is right after you meet Bourbon and he's taking you through the tunnels telling you to keep your light foward. that tunnel has a side tunnel that takes you into another tunnel that is caved in on both sides but has another side room. theres a dead body with a pneumatic gun.

2. the next one comes in the level where your in the lobby of a trainstation and bourbon tells you to search around because the scavengers tend to have hidden caches. the pneumatic gun is in the kiosk just to your right as you start. just walk up to the kiosk and aim your view into the middle of the kiosk and there it is.

or if you want you can pay for one with trade ins at the market level when you get there.

Tikhar locations:
Can be found during Lost Tunnels, near the beginning.
Can be found during Dead City 1, in the ranger cache.
Can be found during Dead City 2, in a dumpster.
Can be found in a broken train car during Frontline.
Can be found in Defense.
Can be found during Black Station, in several wooden caches.

Find a Tikhar early and start killing enemies with it. Make sure to purchase a lot of ball bearing ammo.

Kill methods:
1.) Use headshots (fastest method). For guys with helmets, one shot should pop them off, and another should be enough to kill them (2 shots total).
2.) Kill weakly armored enemies (2-3 shots each). Guaranteed head shot kills.
3.) Kill nosalises. They go down in 4-5 shots and are found throughout the game.
4.) Amoebas. You'll encounter a lot of amoebas near the end of the game, especially when you go to power up the reactor.

If you start early, you'll probably have this within a few levels. If not, then kill all of the amoebas at the end of the game.