Gotta have plenty of healing items plus using the Bat Harpy's slowdown attack. I also used the spell you buy from the merchant, which does around 1000-1500 damage per cast. It basically went slowdown, cast spell, switch to red jelly to reduce damage taken and heal, slowdown, cast, heal, repeat. For casting the spell I switched to Steam or Rock, since they take less damage and you don't get an attack penalty. If you mess up and get caught in any form besides the ones that are resistant to her attacks, you'll die immediately. Furthermore, if you don't have enough healing items you'll be screwed too.