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.
From launch until the April 2022 update, players could not progress this achievement playing Halo 3 multiplayer matches, as the Headshot medal was not awarded in Halo 3 multiplayer matches (due to the fact that the Headshot medal was only awarded in campaign in the original game) - the only game in the collection for which this was the case. Following the update, Headshots can now be earned in that game as well.