I was able to complete the Master of Overkill achievement in the CoE dungeon SM mode by using a special item that you pick up just before a room with weird lightning ball mist things. All you have to do is aim the gun and kill one. The room is decent ways through the instance just before the final room with a few boss fights.
NOTE: If you enter the room without following the NPC you will die in a matter of seconds to the poison.