User blog comment:ImperiexSeed/ARCHANGEL or LEVIATHAN!/@comment-149.210.105.71-20130514175442/@comment-3481504-20130514181436

You're really going to count that as a demonstration of strength.

Unless I neglected or misinterpreted a dialogue, the Leviathans, inside Castiel, at that moment, were not really physically overpowering him but being so influential that Castiel couldn't contain their power.

I would say that Satan could overpower and kill a Leviathan.