No more room in hell - 15 Gamerscore
Kill 2000 zombies
You will get this naturally doing all the other achievements but if you looking to just get this one I recommend loading up challenge 3 and killing zombies until you get this achievement.
You want to load up challenge 3 and grab the sten gun and ammo. After you grab the sten gun just keep killing zombies and grabbing ammo from the dead bodies. I averaged around 300 kills every 10 minutes.