I don't believe I've ever been in a real fight with Roxie, but she has been attacked. If its a full on ball of fur scuffle, I usually am able to grab Roxie by the collar and waist and pull her up and out of it, then kick the other dog away.
Most times though, Roxie defends herself until I can get the other dog away. She just stands up on her hind legs, mouth open, clawing at the other dog. Roxie is good at keeping dogs off of her, never redirects, and is pretty calm about it. She has never injured another dog, although she has pretty thin skin so has come back bleeding a bit once or twice.
I always carry citronella spray now, so any loose dogs get that in the face and go away. I also have pepper spray, but have never used it and only carry it on hikes. Haven't had any Simba vs Scar moments since I started carrying the citronella.
However, if I am able to tie Roxie to something, I will grab a loose dog if there's absolutely no one around that claims it. I usually do have a few treats and this cheap little slip lead in my pocket just in case. I mean, I have to call the cops and have them come pick the dog up since I can't take it with me, but at least it won't get hurt.
Oh... If a dog is trying to kill Roxie, it's dead. I hate having to even spray dogs with the citronella, but Roxie is mine. I will kill a hellhound if I've got to. I'd prefer to choke it off of Roxie and run for our lives, but if I can't then I will do anything I can.