I hate the fellow thought a head shot was all that would do. I'm not saying I'd do any better, that's for sure. I hope I would do something smart and effective, but I hope I never have to find out. I'm a big fan of a swift kick because my legs are more effective than my arms and hands. And I think blows from the butt of the gun could also help. But if I am all tore up mentally or physically, I might do just about anything.
When I worked in the veterinary clinics, often if a vicious dog came at me, I'd shove my fist in their mouth like a big rock. Just something I did in the heat of battle. Eventually I began to "snatch" their lower jaw buy running my hand in their mouth and grasping their lower jaw very, very firmly. In all the cases I did either, the dogs would just open their mouths as wide as possible and seem to be "out of gear" just like throwing a bull's tail over his back throws the bull's rear legs out of gear well enough that you can castrate him without any kicking.
The above is just stuff I have done in the heat of the moment, not something I am suggesting anyone else do. You may want to try the jaw grasping thing on a friendly dog to see the response. It's really a bit odd. In all the times I did it, it never seemed to ocur to the dog that clamping down would get me too, rather they really just seemed to be overwhelmed by the fact that I "had them."