You should speak to the bully face to face!
Honestly, it doesn't matter if she is a girl or a boy. But being a bully is bad enough, i hate bully's, obviously you cant punch her in the face... So i was you, i would go there and speak to her, make her scared of you, if she is fat, call her fat etc.. I know its bad, but if shes a bully..... then just go full force on her.
Teach her a lesson that she will never forget.
Good luck brah