This achievement must be done with 2 or more players, either locally or on LIVE.
To obtain this, simply use 1 or more controllers (if playing offline) or with friends (if playing online) on Crater and have the account that you wish to get the achievement for wait in the area you spawn, they must not kill any enemies whatsoever. Instead the other players must kill everything and last for 1 whole round... you will know when a round is over as it will countdown how many enemies are left. To obtain this online, grab 3 friends and use the same method as above.
I advise you put the difficulty to easy to make it less harder for yourself. Also be careful when returning back to where the person who is camping for health, the doors will keep enemies out of there but if the door is open because you are near it, the enemies can get in and thus kill the camper.