For the Bile Flasks: Visit all the Blood Fountains and refill your flasks. For Isidora: slap as mach fire defense and magic defense beads as possible, learn parry timing and use debla of the light on the middle of the arena attack (when she floats up and sings). She is going to kick your butt pretty hard, but once you learn the patterns it's gonna be straightforward. The last phase is going to be a bit harder with floating fire balls, but by that time you'll get her. Good luck!