Even if you do -50% Atk, even if you do 100 HP, even if you don't let her buff up, the regular overdrive WILL still kill you from full. Probably not if you're also using the Defensive type Soul buffs too, but in a few turns, she'll just do it again. And again. And again.
And I'd imagine that at level 75ish it's literally "play three rounds (add a round for Mordred buff) and you will die, no exceptions"