There was an anime (pretty sure it was an anime) where the protagonist (or at least the one of that episode) could turn his body into diamond. He would make his fists sharp. The villain was a guy who couldn't be killed because his organs and life force was kept inside his gigantic sword. In the climax he swings his sword and the hero blocks it with his head which he had crystallised. The villain's sword breaks and bursts into blood and he dies

I thought it was ninja scroll or something similar but I can't find anything. Image unrelated