How do I fight this girl and win?

I agree with bagman. That or bring your strength up abit, punch her really hard in the mouth, or even better kick her in the shins (hard!) to make her fall on the ground. This will show you, you're much more powerful and then kick her in the rib (not very hard like you're about to break them or anythin xD) and just leave it at that. You may wanna know more about this girl though. She could go off crying to mummy and get the cops round your house! I doubt she'd ever say anything to you again though :D