Aim for the Head

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, and can be used to very quickly earn hundreds of headshots. The achievement is worth 10 gamerpoints.

Tips
Players attempting to unlock the achievement should avoid playing Halo 3 multiplayer matches, as (despite appearing in the original game) the Headshot medal is strangely not awarded in Halo 3 multiplayer matches - the only game in the collection for which this is the case.