Aim for the Head

From Halopedia, the Halo wiki

Aim for the Head is an achievement in Halo: The Master Chief Collection. It is unlocked by earning a total of 2,500 Headshot medals in multiplayer; this includes both matchmaking and custom games. Spartan Ops missions are considered custom games, as are Firefight matches, and can be used to very quickly earn hundreds of headshots. The achievement is worth 10 Gamerscore.