From Halopedia, the Halo wiki
Airsassination is a medal awarded in Halo 4 and Halo 5: Guardians multiplayer for assassinating an enemy while both players are in the air. Both players must be airborne when the assassination is initiated for the medal to be awarded. If either the assassin or the victim begin the assassination on the ground the assassin will only receive a regular Assassination medal. The medal can also be earned in Spartan Ops, most commonly by assassinating Promethean Watchers, though assasinating Shanghelli Rangers using jet packs is also plausible, though much less common.