how do you deal with people who are your friends one moment and bully the next?

Well there are two things that would explain it. Or she's a jerk, or she may be bi-polar. If she is a jerk, you should not talk to her anymore, because that's not being a friend, that's being a fake person and you deserve better than that. If she is bi-polar, stay at her side and tell her everything is going to be ok, say her you're there if she needs something. I hope I answered to you! :)

The only thing you can change is how you react to her bullying. It might be hard at first to stick up to her, but once you do you'll set the precedent that she can't treat you like she is.Plus, is she really a true friend if she's saying that she wants to fight you? Set your boundaries (without violence) and you will notice a difference almost right away.

Do you think you might have done anything to piss her off? If she has no reason to be mad at you then she shouldn't be acting like that. Does someone tell her bad things about you whether they are true or not?

If she acts that way to you for no reason you need to tell her she either needs to start being a good friend or you will quit being her friend. You deserve friends not frenemies.

Try asking her to talk. Then in private call he out on her behavior. Tell her you consider her a friend and ask her if she considers you a friend. If she says yes then ask her to stop. If she says no then say OK I understand. She probably just talks shit when she is nervous. I am ashamed to say that I was a lot like your friend as a teen and I am very sorry for it now. I was a very angry kid because I had a bad home life. I was unable to act out my feelings at home so I did it when I was out and about. This sounds like the way I released my feeling with others as a kid. She may need your friendship more then you know. Or she may just be a bitch that you don't want to be friends with anyway. Either way her behavior is unacceptable. Let her know that in a kind way.