Perfect Dark Zero

do this at the same time as the 100 sniper kills cheevo. just set up a game with the max amount of bots against you, then perch yourself up on top of the map with the desert where the sniper ammo is, along the wall, and you can pick them off from a good distance with little difficulty. you can also perch yourself on the hill to give yourself more time to aim for each head if you are having trouble.