Mine has been in "The Summoning Pits" on Moon, called "The Untamed". Ridiculous health, constant high damage beams and never resting minion waves. It's not impossible, but if you get there low level and no back-up, better say your prayers haha
So, what's your most hated boss nemesis?
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Probably the summoning pits with arc burn. That's a real pain in the ass