The best fight is one which is avoided, however, we always told our girls that if someone attacked them, they had the right to defend themselves.
One of our daughters had a girl in her class in fourth grade who treated our daughter like her personal doll - she was bigger and taller than our daughter, and would randomly pick her up and carry her around. Our daughter got *really* tired of this, and we told her if she needed to, she could hit or kick the girl in order to stop the behavior. Our daughter found a better solution - the next time the girl picked her up, our daughter leaned back and as loud as she could yelled in the girl's ear, "PUT.ME.DOWN". The girl put her down, and never picked her up again, LOL!