Get a MELEE kill with the Skewer, Ravager, Mangler or the Shock Rifle in a matchmade multiplayer game. (I've added 1st-person views of each below so you know what they look like)
They are the only guns that have a blade attachment on the end of the barrel.
Mangler/Shock Rifle are more common weapons.
Skewer/Ravager are power weapons.
Energy Sword and Gravity Hammer do not work for this achievement.
Shock Rifle:
Mauler:
Skewer:
Ravager: