My husband & daughter are driving me bonkers with their constant bickering. He is 63, she is 12,?

he argues with her and teases her as tho he is her sibling, and they end up going at each other verbally, I have tried everything from ignoring it, leaving the room, trying to play mediator, I refuse to get caught up in it, but that makes it worse, he complains about her, she snivels about him, and I am the one having the anxiety attacks because of it. He thinks I should side with him and pouts if I don't, she thinks I should side with her, I don't think I should have to side with either one, but always feel I am caught in the middle. I have tried to explain this to them both, but NOTHING is working. I have suggested perhaps he should not argue on her level, but he is a little too immature to overcome that. SO, I am open to reasonable suggestions as to what to do here, please no flip answers, I get enough of those around here--thanks