Having tried both Expert and Ultimate now I had the same as CBW. On Expert she was just destroying me with garbage petrify spamming but in Ultimate thanks to the trash mob placement I always get the Medusa with full gauge and all my buffs ready to use, dropping her to like 40% straight away then using abilities to burn her into to stun and that was with my levelling up party by accident as I was doing raids for limit break materials while leveling up non-maxed kamihime and the new souls.

Maybe the problem isn't I'm not powerful enough to do Expert but in fact too powerful (but not enough to just auto-attack through trash) that I burn through the trash too easily that I hit Medusa with only 40%-50% gauge with no abilities ready and petrify just ruins me. Could try not using abilities but then damage builds up too quick and Expert never seem to drop healing items very much.