So I'm going to guess that his defense at the apex of it might also include some sort of damage reflection element, maybe. Or the creature destroyed itself from its own AOE or from exhaustion.
On a side note, should've guessed that he had some form of offense that wasn't related to using magic/spiritual attacks in the form of magical attacks. Just because he has no attack skills doesn't mean that he can't just go to the top of the JRPG action menu and just spam "ATTACK".
And if no attack can land on him, at worst it just becomes a long drawn out match as he just pokes the enemy to death.