What do you when the narcissist and their flying monkeys start verbally assaulting you all at once?

I would say take a look at what changes you can make because you are probably not going to be able to have them see your viewpoint, they have already sided with the other person, and if they are verbally abusive, they are not emotionally mature individuals. So, take a look at your situation and what changes could you make. For instance, if they are verbally abusing you in person, could you change having to be in that location or could you report this to the police? Sometimes that is hard to do, but some flying monkeys will stop right away if there is going to be a legal document. If it’s on social media, I would make my page private and block anyone who is common between both you two. It’s easier to work on you and heal but cutting off all ties possible or limit your interaction with them. And most definitely don’t give them your energy. You have to set a boundary not to put up with abuse, period.