I'm not sure if I'm reading this correctly. So your boyfriend was cheating on you with another girl, and she found out about you and she wants to fight you and not him?
I'm not going to say anything about the whole cheating situation, but it's probably better to avoid these situations in the first place :/
Anyway, I would try to avoid this so called fight. She's probably going to scratch your face, punch/slap you, grab your hair, call you a *****/ho/etc, and shove you around. Of course you could do the same too, but that wouldn't be smart. These things get no where.
And if you're worried about being perceived as a weakling, just remember that you'll most likely be known as the "*****y cat-fight girl" if you do accept this "fight".