King of cumans Invincible

I am on the 3rd brawler which is king lf cumans in the quest where you brawl in Skalitz. Even when cheesing with stabing while clinching, his health will not go down. Is this quest related? Is he just that tough you dont see his health drop much? Or is this a bug?

You need to drain his stamina, give him proper thrashing with no time to recover, when his stamina is gone, his health goes down quickly.
Pro tip: when he is unconscious, take his helmet off, thank me later.