This one is a staple in all the Borderlands games. What you want to do is look at the guns in your inventory and keep an eye out for one that has a knife attached to the end.
Once you have one of thee, equip the gun and melee an enemy (to death). Once you kill one enemy this way, the trophy will unlock. It’s worth noting that you don’t need to kill the enemy in one melee attack, as long as the killing blow is with the knife ended melee you’ll get this one.