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Mine was definitely Pontiff Sulyvahn, took me a while to beat him. Probably because I was using a +4 long sword but whatever :p. I haven't beaten the game yet so I obviously don't know all bosses yet, there's probably a harder boss than Sulyvahn :p
Edit: Nope Nameless King is harder, holy shit it feels like it's impossible
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Wolnir, solely because his area is [i][b][u]Hell.[/u][/b][/i]