F.E.A.R. Files

You have to kill ten critters (rats/cockroaches) in one play through. There are only 14 in the game, spread across 4 locations:

1. There are 3 rats at the beginning of the game. You'll enter a tunnel and have to duck under a grate. The three rats are immediately behind the grate.
2. There are 2 rats after being split up with your buddies and taking out the first couple of enemies in the sewer in "Interval 1, Underneath" (one of them is carrying a grenade launcher)
3. There are 5 rats [2+3] in two adjoining dark rooms full of crates towards the end of "Interval 4 - The Deep"* "Interval 4 - Buried", shortly after confronting the invisible enemies with red eyes in a large squarish room that is flooded.
4. There are 4 cockroaches while leaving the subway (this is not much further than the previous location). as you are climbing the stairs, on the left hand side is a civilian corpse on a bench. as you walk by, music plays and the cockroaches scurry away.

I've also made some images of the locations of the critters as well as the locations immediately before hand so you should be able to recognize them easier while playing which you can find here: Album of screenshots

The achievement will unlock immediately upon meeting the requirement.

*Thanks WheeledWinter

I just wanted to say don't try killing yourself at anymoment and killing the same rats, it will not keep the count upon death. (I tried it lol)

I actually found 3 rats during "Tutorial"
2 Rats after being split up

When you get to interval 4, the second level there will be a room filled with water, then after the water filled room there is a generator room that has 2 rats that were behind a generator.

After the generator room is the room filled with crates that have 3 rats, This is where I got my achievement, which you will too if you get all the rats listed. Others say cockroaches count too, incase you miss any of these rats.