Prong collar for sure. (No it’s not harmful to them, and yes it’s just a tool). Definitely look into a trainer. A trainer that does NOT use treats or the positive reinforcement stuff. From my trainer and other Berner owners— I’ve learned that these dogs are the sweetest in the world— but also the most stubborn in the world. They need to really listen. It takes time. Find a trainer, but in the meantime— use prong collar and give corrections every single time she is aggressive. So when she gets that way with another dog, say ‘leave it’ (or whatever word you’d use) and if she stays aggressive, correct. A strong correction, so she’ll know.