Achievement - (Secret) Copper Stopper unobtainable

I have completed every single quest, and killed the Copper Monstrosity multiple times but this achievement will not unlock. I have 31/32 (980gs) achievements but I just miss this one. I completed the entire game. Majority of the game played single player, aside from first few hours a friend was in my game.

Steps I have tried:
Copper Monstrosity Revived quest, multiple times

Restarting game

Reloading game, re-killing Copper Monstrosity