Shadow Warrior 2

This achievement, as well as the other kill acheivements, is cumulative, just kill enemies until the achievement pops. It doesn't need to be done in one sitting, and any humans, creatures, robots, even the bunnies you see in some levels count as kills. The Xbox One's achievement tracker keeps decent progress, but if you want to see your exact kill count, you can check the Challenges section of the pause screen or main menu for it and progress for other challenges as well.