The paradox lies within the term, "great bodily injury." Does a victim have to wait until after he's sustained an injury that manifests in to great bodily injury or death when at the time the injury was inflicted he lacked medical knowledge to know the potential seriousness of the injury? The benefit of doubt has to go to the innocent victim.
Preexisting physical infirmity has to be considered. A blow to the head of a man with a preexisting brain injury could be fatal. Only the victim would know that a punch to his head has a high probability of causing his death. Hence, he would be justified in using deadly force to protect himself from a punch to his head that might cause only minor damage to a person without a preexisting brain injury.
Zimmerman's head was repeatedly slammed into asphalt. It appeared obvious that Martin had intended to inflict great bodily injury upon Zimmerman.
Then there's the guy who was sucker punched as was depicted in a recent Antifa marauding video. The man was unconscious on his feet. He fell to the ground. His head slammed upon asphalt below, which was likely to cause great bodily injury or death.
California Penal Code Section 692 clearly states that a victim does not have to allow an assailant to inflict injury upon him before he can respond appropriately.
How is a victim to know an assailant's intent? The best way is what an assailant says before he attempts his attack. If an assailant were to say, "I'm going to kill you," a reasonable man would use force necessary to prevent his murder or great bodily injury.
If direct, physical, and scientific evidence supports a victim's reaction to an unlawful attack, he should be presumed to have acted in self-defense. Again, the reasonable man standard applies.