From Halopedia, the Halo wiki
Longshot is a medal available in Halo 5: Guardians multiplayer.
Longshot counts as a Weapon Proficiency Medal, and can be earned with any precision weapon and any gamemode. Most commonly, it can be earned during Arena matches with the Magnum, Battle Rifle, and DMR.
To obtain a Longshot medal, the player must kill an opponent at long range with a precision weapon. Despite the precision weapon requirement, it does not require a headshot, though it will count towards both medals.