Back to the evidence of the case, briefly, it now appears pretty well settled that Martin was on top of Zimmerman. That's the eye witness of Good, who had the closest view and most accurately described GZ and TM. It's also consistent with GZ's injuries. It's also consistent with GZ's back being covered in grass. It's also consistent with the position of TM's body. TM was shot in the chest, but he was found face down with his arms under him. So unless GZ shot him while standing above him, then rolled him over onto his chest, he was shot while above GZ and then fell over.
The only witness to claim that GZ was on top of TM made this judgment only on seeing shadows. She assumed the larger person was on top, and then assumed that GZ was the larger person based on watching the news and seeing pictures of TM in a hoodie (face only) and a football uniform (when he was 12 or 13).
So for the state, it seems they've only got a few routes to conviction. They need to convince the jury that GM started the fight (he deserved it), that GZ is a racist POS (eff the law, he deserved it), or that despite GZ getting his crap kicked in, lethal force still wasn't reasonable (seems unlikely, but jurors have done crazier things).