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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The Devil Walker in the first strike during the Alpha. It took me and 2 other people about 15 minutes to kill it. The thing was a monster before the beta.