Perfect Dark Zero

Set up a 2 person local map and choose desert as the map, sit on the balcony to respawn ammo and move the second controler to the street, you will get instant reloaded ammo and easy to bring the second player to the kill zone.

This achievement can be earned offline. A more fun, less grindy option is to go into Kill Count > Deathmatch and set it up where you have 15 agents in the map. Set the bots to Agent difficulty. Now set a score limit to 100 and no time limit. It will basically be a free for all and the game will end when someone reaches 100 kills. You don't even have to be that good at the game to accomplish this. Good hunting.