Needle Combine Kill
From Halopedia, the Halo wiki
The Needle Combine Kill medal (as it is known in Halo: Reach) is awarded in campaign, multiplayer, Firefight and Spartan Ops when a player scores a kill by filling an opponent with enough needles for a supercombine detonation from either a Needler or a Needle Rifle. It is known as Super Detonation in Halo 3, Needler Kill in Halo 3: ODST, and Supercombine in Halo 4 and Halo 5: Guardians. In Halo 5: Guardians, the medal is only awarded in Warzone.
Causing a detonation requires an enemy to be shot with seven rounds from a Needler or three rounds from the Needle Rifle.