You can get this easily in the beginning of the Dark Harvest level. I believe it is the 3rd chapter in the Egypt levels. You'll start off by falling into a pit and Gnaar's will fall into the pit after you. Just shoot them all as they come down to build a hill of their dead bodies for you to climb and get out. I don't think this achievement is missable, I got it here without even trying since a lot more than 50 fell into the pit.