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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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Crystal. -blam!-ing. Sage. Now let me explain. I'm a pyromancer, so for the first 20 attempts I thought it would be smart to purely use fire to kill him. But eventually I learned it was retarded, so I used a mix of sword and fireballs to kill him. But honestly though I can't beleive how hard I was raging, screaming