From Halopedia, the Halo wiki
Perfection is a medal awarded in Halo 3 and Halo Reach for winning a Slayer game with at least 15 kills, and without dying. It should be noted that you do not need to get a Running Riot, 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. Although the medal does not appear in Halo 4, there is a "Perfection" Commendation which is advanced by the same criteria.
The medal is represented by a black hexagon outlined in a blueish steel colored hexagon with a smaller blue hexagon in the center.
One tactic to obtain the medal is to camp and avoid enemies. When they advance, stay out of sight, and as they get closer, execute an ambush or assassinate them. Players can also try to perfect ranged attacks like sniping, or can just play the game recklessly and hope to get 15 kills without dying. Camping and staying away from enemies can often result in a lack of kills, but you are more likely to stay alive for the whole game, so a mix between camping and advancing is good strategy. Perfection is considered one of the hardest medals to achieve.