Maybe that sword being broken save him. Or else if he killed Tara they will summon WolteKamui and no one can escape alive
Also, His journey initially led him to Dragon Hunting Flash mean that this is not his true ultimate skill?
It is lucky that he didn't infuse all his power into his sword like Crimson ordered and infused some into the princess's sword. If his sword become anymore strong maybe Kamui would be summoned already.
I bet even Crimson didn't expect he would still be this strong with his body in this state