Warhammer 40,000: Shootas, Blood & Teef

This achievement should actually be kill 500 Greenskins as Gretchin/Grots count as well. Although you'll likely get this throughout the game (Crash Landing, Goff Settlement, The Goghamma, etc are full of Greenskins), the best level is The Goghamma Part 2.

This is because upon starting and floating through space (don't ask how you can somehow breathe), you enter a room with infinite Gretchin/Grots. Instead of rushing through and turning the valves, just keep killing until you have the achievement.

Video below of the level start and the room (credit to Dreemax for the video).