You get story progress (not yet but in future updates) some sexy CGI and, the boss weapon.
If you find her too difficult to fight, rolling and using the cursed sword is the best strategy for the first phase .
For the second phase, I recommend to use the Whip and hit the Boss Hammer so she drops it, you can also parry it on the air i think.
And of course, a lot of knifes and food (or the special skill of the Vine Bow)