[Suzerain of Sheol]

I was stuck on her for a while, but I found using the armor you can buy from the turtle vendor that increases your i-frames, and using the Sea Tea he sells which increases your heart's movement speed, made her really easy. I beat her the first try after I went back and bought those. Don't get the weapon he sells though, it's weaker than the ballet shoes and the i-frame buff doesn't stack supposedly.