Well i beat all 3 avaible stage she have right now cause the 4th one is only unlock once we reach the capital so much much later not even in game yet si anyway.
I grind vili until i was high enought like around 30 each stats and you must have the "demon layer" or its gonna be way harder than needed i also had the hero cape and use cleanse lust everytime it reach too high regen your stamina with item if too low and keep spaming it if its still toi hard check the wiki about her fight its gonna help you more than i do.