Second Quest on the Nuka World DLC. Available after "All Aboard".
Just fight your way through the gauntlet. Kill Overboss Colter with the Thirst Zipper (that's the little squirt gun in the room just before you enter the arena). Whenever you see the suit sparking with electricity, "squirt" Colter a bit with the gun. Once you've hit him, you have a small time window to deal damage. Repeat until he's dead.
Speak with Gage and the quest is completed.