History of Boop

Boop requires you to "Defeat a Guardian with a melee ability in Rumble in the Crucible."

How to complete

Rumble is a 6-player free-for-all Crucible mode, so you only need one melee kill on another Guardian. Fights are crowded and everyone is hostile, so look for opponents weakened by someone else's fire and finish them with a melee. Prefer corners and ammo-crate areas, where fights concentrate (per Destinypedia's Rumble tips).

Unconfirmed: I found no guide specifically covering this triumph, so whether weapon melees or abilities-only count, and any reward, are not verified. Sources describe the mode only.