Dim Mak is an achievement in Halo: The Master Chief Collection. It is unlocked by earning a total of 250 Beat Down medals in multiplayer; this includes both matchmaking and custom games. Spartan Ops missions are considered custom games. Assassinations earned in Halo 4 multiplayer and Spartan Ops do not count toward the achievement. It is worth 10 gamerpoints.
Dim mak refers to "touch of death", a martial arts technique that is said to be able to instantly kill the opponent with a very light blow or even a simple touch.