While the Grand Mastreo may be twisted in body and spirit, he can still pack a mean punch. Bringing the pain to someone as base and low as a mere human mage is something he can do with his giant, bloated form. Pounding at the ancient mage with his deadly status attacks and giant limbs is going to be a joy, one that the oft put upon Mithra will not enjoy. While his foe can heal limitlessly, he can't do much else, and he'll slowly get worn down by wave after wave of diseased flesh crashing into his body. It'll be a death that fits how he orginally left the world of Mortals- brutal and senseless, just to appease an evil tyrant.