If you're relying on luck you're doing it wrong. The fight can be beaten with a combination of overwhelming attack power and status affliction guards. You should be able to get two uses of Amon's skill, especially in Ultimate. That's enough to Full Burst at the start of the fight, use skills to reduce the Rage meter to 0 in the next turn, and then pound on Medusa until stun breaks, at which point she should be nearly dead--all while guaranteeing that Petrify never lands. If your attack power isn't sufficient, then try the next difficulty down. As long as you can at least do Standard, you can get something useful from this event.