SlyOW
They mainly fought within Mundus, where her strength is lowered. Daedric Princes are almost omnipotent within their realms, but since they escaped Mundus at its creation without putting in any of their essence, they are weakened while in it. Also, God was constantly attacking her on the other side to retrieve Alex. She was essentially fighting Alex, God, and Mundus at the same time. And she did win the fight, Alex only managed to flee.