No, he doesn't have to prove that Tray approached him. He would have to merely prove he was somewhere he lawfully had the right to be, and was in reasonable belief that he was about to be maimed or killed.
His injuries back that up, IMHO. He doesn't have to prove that Tray was the first to engage in the disagreement.
my bolding
That would apply to Trayvon, not GZ.
JMHO