I think the issue is just that he can be cheesed very easily. First time I fought him, I mistimed a dodge and he basically combo'd me to death. Second time I basically stunlocked him with a greatsword. Third time was on my next character, a caster. The right caster build can melt him in mere seconds, which is exactly what happened. Fought properly he is indeed possibly the hardest fight in the game. But he is deceptively squishy and easy to stun for the 'final boss', so certain builds or weapons can destroy him with relative ease.

just use any tactic that doesnt involve getting hit and you should be fine.