I would play act some scenarios that have happened and help your daughter to come up with solutions. You can start by using dolls, and then move up to actually acting it our yourselves. Take turns being your daughter and the Mean Girls.
We do this with my daughter whenever there are some bad social situations. It seems to prepare her and she feels more confident.
She was getting picked on at school by a boy. The boy would pick on her until she got mad and lashed out. The problem was, she would get in trouble for lashing out! And if she argued with the boy, they both would get in trouble. We didn't think it was fair that he would pick on her and then she would get in trouble for defending herself. Multiple talks with the teacher did not help as this was happening on the playground during recess when the teacher was not there and there were only lunch aids helping.
By playacting several solutions, she did learn how to ignore the boy and walk away. But not just ignore, she would walk away and start talking to someone else.
We also taught her the right way to tell an adult. You don't go up to an adult and whine that someone is being mean. You tell the adult exactly what happened in a non-whiny tone. Not that your daughter is whiny, but I'm just merely saying that's one of the things we drilled.
I feel bad for your daughter, I hope she can stay strong! Girls pick on those that they can, so you daughter can keep herself from being a victim by staying strong.
ETA When someone said something mean to or about my daughter, we've taught her not to say mean things back, but also not to ignore because usually the person just keeps saying it until you respond. Instead, my daughter looks that person dead in the eye and says "Why would you make such a rude remark?" It seems to work the best! She's confronting the person's remark, but calling them out on it in front of everyone else. Usually it's enough to make the child who said it pause, because it's not the response they were expecting. They were expecting a rude remark back (which is part of the game) or ignoring. In both cases they are free to continue to ridicule. The important part is the look on your daughter's face, it needs to be practiced. She needs to be able to look that person dead in the eye and say it with confidence.
If the other person says "Well, you deserve it!" or "Well, it's true!" then your daughter, with the same seriousness, can say "No one deserves to be treated this way." and then walk away.
I GUARANTEE, if done correctly, it works. It even works on adults. This suggestion was actually adapted from Miss Manners' suggestions on how to handle rude adults.
What it does is it calls the bully out, without being mean. Most bullies do not want to be exposed in this manner, and usually they will stop.