Domination (achievement)
From Halopedia, the Halo wiki
Domination is an achievement in Halo: The Master Chief Collection. It is unlocked by earning a total of 100 Legendary medals in multiplayer matchmaking. The achievement is worth 10 Gamerscore.
There are six Legendary medals: Extermination, Killionaire, Perfection, Steaktacular, Triple Double, and Unfriggenbelievable. They can be found on the top line of the player's in-game Medal Chest.
Tips
Steaktacular is arguably the most easily earned Legendary medal, and is most easily earned in Big Team Battle matches, as winning by 20 or more kills is much easier in 100-kill matches than in 50-kill matches. Triple double is likewise more easily earned in in BTB, and a similar argument could be made for Unfriggenbelievable and Perfection. The only medal which is irrefutably harder to earn is Extermination.
If the player is sufficiently skilled, they can earn many Killionaire and Unfriggenbelievable medals in Infection.