Quirrel's main problem was that his idea of muggle weapons technology was a machine gun. If that was the best Harry could do, he would have lost.

His other problem was giving Harry access to his wand for more time than strictly necessary. Had Harry figured out what to do earlier, he wouldn't have even needed partial transfiguration, since his wand was touching someone else's during the Vow. But Quirrell knew better than to rely on his plan being perfect. He should have known to minimize the time during which Harry could act.