Metro 2033

As easy as it sounds, you will get this achievement simply by playing the "Defense" level in Chapter 4.

After the big fight where you have to defend the tunnel area and you are given the cassette tape, you will go through that area (with bunks, chests, etc.), past the small railcar and past a wooden walkway on the right. You'll then reach a lit area at the end of the tracks that is blocked/barricaded with sandbags.

If you climb up the ledge to the left and venture around the corner to the left, you will see the shadow of a Lurker around that corner. Once you alert/kill him, there will be plenty of his friends throughout the short level. They come out of holes in the ground and in the walls. A shotgun to the head works nicely (especially if you have the one you get with the Gamestop preorder).

Oh, and be careful you don't step into one of their holes. You'll die.

UPDATE: Based on comment from TSG Dwaggie, you "have to be in the MAIN room. You CANNOT be in one of the small square side rooms, otherwise they'll stop spawning after five or six."

As THEICEMANCOMETH stated, you can find Lurkers in Chapter 4 - "Defence" (and also in the following chapter "Child"). These creatures are extremely fragile, I used my Kalash and was easily able to kill 3 - 4 per magazine.

In the section of chapter 4 where you have to wear your gas mask, I recommend sprinting up to the high ground (there should be a staircase up out of the gas on the right) and removing the mask, this serves two purposes - One, if a sneaky Lurker gets a suckerpunch, you're less likely to get your mask fractured/broken, and two, it's the high ground! there are no lurker holes up there, and you can hold the staircase nicely. Patience will net you several kills here. (I infact sat here until the lurkers ran out, and reloaded my checkpoint to do it again. Got it on the second run.)

Edit: I'd also like to point out - reloading your checkpoint is absolutely not necessary. You'll encounter a lot of these guys.

And hands up who fell in the hole on purpose because they're too inquisitive for their own good? =P

Lurkers are mutant rats.They are pink and are skinless, they usually dig holes and pop out of them without warning. They die quite easily but tend to run.
On the level child there is quite a lot of lurkers around the place,
***spoiler***
also when you return to the cursed station (where you blew up the tunnel) this area contains a lot of these creatures.
This achievements will come naturally as long as you don't run away from them.