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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Any public event where the "kill this guy" is shielded. If you're solo and/or unprepared they can be a real pain in the asshat. The shields recharge so fast. Even more so when it's a captain with his full auto never ending ammo shotgun.