It might not be that difficult, assuming they get an indictment. Brown's companion's testimony won't be much help. The two women witnesses should provide a pretty solid narrative. I think it will boil down to: a) is Officer Wilson going to claim he shot Brown because Brown was charging him, b) can the forensics establish whether Brown was running or standing still when hit and c) did Wilson have the legal right to use deadly force on a man charging him if the man is unarmed.
As for b I think a lot depends on the bullets' path through Brown's body. If they're all straight through then it will indicate Brown was standing straight up when shot. If they're slightly angled I think that will indicate he was bent forward and running. But I still think the case depends on c. If an officer does have the right to shoot an unarmed man who is charging him than I don't see Wilson getting convicted.