The Perfection medal is a medal awarded in Halo 3 and Halo: Reach for winning any Slayer game with at least 15 kills, and without dying. It should be noted that you do not need to get a Running Riot medal, just 15 kills, which means the medal can still be obtained if the game selects a new host and resets your spree. If you are in a Team Slayer match, you have to get 15 kills by yourself without dying and your team still has to win.