Diabolus is not intended to be attacked directly though, and our characters aren't city blocks. It's like a Shadow of the Colossus battle, his great strength is offset by being cumbersome. He can be defeated by characters of the same power level as required for the other demons, as long as the characters are suitable.

Edit: I suppose we just gotta experiment around. I've got characters suited for this power level and for a higher one, so in the end it's up to Laios.