From Halopedia, the Halo wiki
Decorated Warrior is an achievement in Halo: The Master Chief Collection. It is unlocked by earning at least 10 different medals in one non-team Halo 2: Anniversary multiplayer game. The achievement is worth 10 Gamerscore and can be unlocked in both matchmaking and custom games. It also unlocks the "Magnum" Multiplayer Emblem.
The achievement is a nod to the Decorated Soldier achievement in Halo 2 for Windows Vista, which required players to earn 8 different medals in a free-for-all multiplayer match. Despite the increase from 8 to 10, the sheer abundance of medals in Halo 2: Anniversary actually makes this a significantly easier achievement.