If your playstyle does't involve a lot of prosthetic tools/items, this miniboss can be one of the hardest in the game. She's optional (unlike Shirahagi in the swamp later on), so run past her and continue on with the Gun Fort. Fight her later after unlocking the idol after the Gun Fort, or much later when you have more HP.
I'm on an NG+4 run and Shirafugi continues to be one of the only minibosses that surprise me with how tricky she can be. Her hits can usually be blocked/dodged easily, but missing even one can be a death sentence.