Tiny Tina's Assault on Dragon Keep: A Wonderlands One-shot Adventure

This trophy can be obtained in Hatred's Shadow as soon as you first arrive there. When you get to the main entrance of the keep, head on to the right of it and look for a large shadow against the stone wall. This shadow called "The Darkness" will have a health bar when you get close to it. Now you have two options available. You can either kill it with a gun to get the trophy, or kill it with a Magic Missile grenade to get the treasure it is guarding. Unfortunately, killing it with a Magic Missile will not unlock the trophy. Luckily, leaving the area and coming back will respawn it so you can get the gun kill then. There have been some reports of it still not unlocking, so you may have to try it again on another playthrough.