If you don't pick this up just playing through the game the first time, Load up chapter 19 on easy.
At the start of this chapter you'll have to pull 2 switches to open a door, after you've pulled both the switches, if you stay in the main room, and don't go through the door you've opened, enemies will just keep spawning to no end, There is a lot of ammo on the ground if you need it, Also if the enemies get close, try firing from the hip, it is really effective. After your 50th kill in a row the trophy will pop