User blog comment:IamJakuhoRaikoben/Chapter 356: Tartarus Arc, Review./@comment-5303512-20131019065631/@comment-7418318-20131020091515

Exactly because he is so seeming invincible (against anything that's not a Dragon or sucking out his magic anyway).

It would make the enemy who managed to overcome his monsterous strength seem all that much more threatening and thus it's a good story device.