How the hell do you kill this guy? Me and my friend will get him down to a low health and he will just become a Black Hole Thresher and down us, while we can't do anything to counter it. How do you stop him from doing it?
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If you don't have a Siren to pick you back up, your two choices are to either deal enough damage such that he retreats underground, or sprint in the opposite direction immediately after he creates his black hole. If you happen to be right next to him when he does, you're pretty screwed.