Quantum Theory

The online for this game is dead. You might come across 2 or 3 randoms at times, but you need a full 8 player game to unlock this.

Get a boosting group together and set up a ranked Deathmatch and make the map The Great Hall. Once it starts up have everybody make their way to the bottom floor and have everybody who is getting killed to bunch up in the middle. The person who is killing then gets headshots on all of them to make things go faster. Once the person kills everyone, cycle to the next person and continue doing that until everyone has had a turn.

This achievement will come naturally if you are boosting the rank achievements with an 8 person group. The achievement will unlock at the end of the game.