Obviously there are more such as vermivorous and the son of crawmerax however i did not include them in the poll as there are only 7 slots if you thought either one of them was the hardest comment below. Note for vermivorous we are not including the difficulty of spawning him just the fight itself. And for all these fights assume there is no merging, glitching, or exploiting.
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Master Gee. You can't even fight him.