The law doesn't require a vicious beating to merit lethal force in self defence. What it requires is a situation in which there is a reasonable belief that death or serious injury can occur. Reality is that one punch or blow to the head can be deadly or cause serious injury. All the defence really need to do to show self defence is to so that Zimmerman was there legally and was experiencing blows to the head from Martin when he fired.
As will become apparent during deliberations, this will be the crux of the case, however as was pointed out in the older old thread by a at least one self professed lawyer, there is much more to self defense that just 'I had a reasonable fear of loss of life or serious physical injury', and that is where the deliberations are possibly going to be very complex. The actual setting of the events and the reasons for the confrontation do matter.
So if the jury decides that GZ was attacked without provocation, and that the acts of TM would lead a reasonable person to respond the way GZ did, then the self defense case will be easy. But as was pointed out in the old old thread, there is much more to a self defense claim than that.
So while the prosecution may seem to be failing right now, there is a long way to go, they are likely to try to establish that GZ did influence the attack by TM, that the choices made by GZ were not reasonable, that he contributed to the escalation of the confrontation and that the injuries being received by GZ were not life threatening or likely to cause serious physical harm.
Then it really comes down to the jury's impression of what happened and how they weigh all the facts, if they just focus on GZ being beat and feeling threatened then they will rule for self defense, but if they feel the other factors are there (which will be solely up to them) then the deliberations will get very long and drawn out with lots and lots of instructions.
And that is where the two narratives will matter.