I defeated the Stain Princess! It was so much fun trying to figure out all the different attacks. Some were more telegraphed than others, and each had its own intuitive timing for invincibility frames. For me, the trickiest part to git gud was distinguishing between Biboo's spinning attack and the quick slash. They both start with her moving her sword to the right (her left), but the timing for dodging is totally different. In the end, I just gave up on trying to dodge perfectly and opted to move away whenever either of those two attacks started.
But overall, it’s a well-designed boss fight! (And it even has a second phase? What?!)