The battle is still rough but it is beatable and required less attempts than the onis. Every battle is beatable and this is the intended way, abusing that one weakness she has. I think the battle is intended to be the gatekeeper and first proper challenge to the player in the story (Venefica, Sofia, Oni Twins are optional) that is mandatory and does tell the player to go back and do other stuff before coming back or figure out how to beat the boss (figuring out her weakness).
I like that. I think I have an easier time with her compared to the optional boss battles because my willpower is still level 10 and not looking too good, especially against Sofia and the Onis.