Fight the warboss, near the coastal fortress. There's a red fortress. around here
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It's hard. the jump attack he does has some last second aimbot bs. Try to get the timing right, wait around 3 seconds, then sprint. I killed with the rag clothes + 12 frag bombs + like 10 potions in a nymph playthrough. I'll probably never do that again lol.
Or you can just be a mage, and make a 4x4 square of the red tentacle things around him. Let the tents draw aggro, While constantly making more tentacles. EZ.