This achievement should come very easily by the end of the game. Nosalises are the four-legged creatures you fight throughout the game. Killing them in any fashion (firearms, melee or otherwise) will count towards the achievement. If some how you manage to beat the game without killing enough, you can load up any mission that has them. Two notable missions for this are "Defense" during chapter 4 or "Cursed" during Chapter 3.
"Cursed" I think is the better of the two because it seems like as long as you don't complete the objective they just keep spawning. Khan will tell you to go find a bomb, and he will stand by two soldiers. Go straight down the platform from here, and on your right you will pass three steps of stairs, go up any one of them and you will come to some escalators, which are destroyed. The nosalises seem to be coming from here, so just stand back a bit and wait for them to keep spawning, and kill until you have your achievement.