How do you save your dog?

Hi everyone...I'm still really shook up by what happened at the dog park on Saturday and I had a nightmare last night of a repeat...I just feel really terrible as a doggie parent, and need some advice from loving and educated doggie owners who have some wisdom to share :redface:

What should you do when your dog is being bitten/aggressively wrestled by another dog who is larger, and considerably more dangerous than what you're used to handling?

Just some background info - small Bichon (my dog) and a Boxer (mix?). He was pinning her down, and biting her ears and tails, and she was crying out loud in pain, and I tried to get between them to push him away with my legs, but I couldn't because he had a hold on her and it was hard to push him away, and so I was screaming loudly "STOP STOP!" hoping he would back off, and I asked the owner to extricate her dog but she told me she won't, and that it was my problem...and then finally my friend who's a police officer was able to use a very commanding voice, which scared off the boxer. If it wasn't for him, I don't know what I would've done, and half of me was so terrified I felt frozen in fear.

Since I was screaming STOP STOP! all the other dog owners in the park were staring at us (and there were a LOT of dogs that day), and the owner finally put the leash back on her boxer and left the park about an hour later. I had Butters on a leash after this incident to better manage in case it were to happen again, but she was getting along really well with the other dogs in the park, and played well with them (I don't know how shook up she was about it). Before this incident I knew I had to be wary of him as he was jumping on me and my friends and mugging at our treat bags trying to get some food from us, and the owner did nothing to correct his behaviour.

I don't know how this erupted and it all happened so fast, that I don't know what lead up to it - Me and butters were previously working on our recall in the park, and when she'd get a few feet away I would call her back to me, and when I was giving her treats, he would always be there too hoping to get some as well, so he was always kind of close by,

I just need some advice on how to better handle the situation so next time it doesn't happen again, and if it does, that I'll be better prepared to resolve the situation.

Please only constructive advice and tips/warnings, just want to know what I can do next time as I'm still really shaken up about it. Thanks!