First of all you want to be in a PRIVATE match with 1 other friend, from there the host selects Legends and killers Free for all Matches. Once you are both in a match that has to be SHOOT-OUT, (If it is not shoot-out, vote past maps until you do get the shoot-out game mode). Then all you have to do is get 5 head shots on your partner when the game starts. The achievement will pop in game so don't worry about finishing the match.