I think that attempting to shoot him there wasnt giving an intelligent enemy very much credit. It would only work if the stupid mistakes that Voldemort was making were real, and not a ruse. Given that Harry possibly has only one chance (because Voldemort promised in parseltongue not to try to harm Harry unless he tried to harm him first), taking the first opportunity that presents itself, which might be a trick to get Voldemort out of that promise, is probably unwise.