If one guy is trying to beat you, justifying the use of deadly force could be very difficult indeed.
It could be. On the other hand, it might not be. That depends entirely on the details of the specific situation, and how much some people will "muddy the waters".
If the "disparity of force" is obvious (200+lb teen vs 105lb granny) most people would not think long about accepting use of deadly force in self defense as a viable argument.
When the individuals are more closely physically matched, a lot more doubt can ensue.
I've heard people so thoughtless as to actually say "why did he have to shoot? He was only getting a beating!"
A lot of us are no longer young, or physically fit, the beating that someone else might recover from in a few days might be the last event in my life, and I, personally feel it very reasonable to use deadly force as a last resort to avoid that.