Perfect Dark

All you have to do for this achievement is get friend on XBL or in offline splitscreen and complete any of the levels on any difficulty (all levels in a mission). I would recommend Carrington Villa on agent as there is only one level and it is quick.

Also to speed up the process have the counter-operative go round killing all his own men so it’s quicker for Joanna, then swap round. (Or the counter-operative could just take a suicide pill for every guard he becomes.)