After something like four hours of trying to beat him and getting increasingly angry at the game for having such a bullshit enemy with the worst aspects of Dark Souls bosses, so much so that I was beginning to dislike the game, I just got fed up so I broke the rule.

I downloaded a cheatengine table and gave myself invincibility and the ability to instabreak enemy's posture. Rampaging through the area with the enemies and killing everyone in one hit, only to kill the bastard himself in five seconds gave me more pleasure than killing him the normal way ever would. I can't remember the last time when I felt such catharsis in a game, it was incredible.