A kid was harassing my 10 year old daughter on the school yard. I authorized her without her mother knowing to take care of the problem. Showed her how to smash someone flat in the nose. Also told her she could be nasty and aggressive in kind. Told her she would not be in trouble at home. She solved the problem one way or another. I never got called in. I think knowing I'd back her up was a big help to her.
For serious physical harassment, there's always a short length of 2X4, a head cover, darkness and concealment.
Far too many people on these threads assume that their kid can whip the bully any time they choose to. It seems your child was one of those. Unfortunately the opposite is more often the truth.