If you have completed the DLC story you do not need to start a new save file or enter a newgame+.
If you travel to Negul Neshai and make your way to the boss room at the end. When you start the boss fight Deathtrap will be in the middle of the room as an ally. The boss will frequently cast a shield which causes enemies to spawn, Deathtrap can one shot these. Once the shield has been depleted Deathtrap will have a chance to target the different enemies which are counted towards the 50 needed. I tried playing around with the positioning as it seemed to target the boss mostly, there's a higher chance it will target the adds if they're close but it was a bit random. I didn't do any damage to the boss and only killed an enemy if I entered FFYL. After about 20 minutes the achievement popped.