If you want to make the scenario more realistic then put more time pressure on Voldemort or put him under more cognitive stress some other way. The hardest part for Voldemort is solving this problem in a short time span and NOT coming up with a solution that foils Harry. The reason experienced soldiers/gamers with little to no intelligence still win against highly intelligent combatants with no experience is that TIME matters when you're limited to a single human's processing power. In virtually every combat situation one is forced to make decisions faster than one can search the solution space. Only experience compensates for this deficit to any measurable degree. In this situation there are several aspects that Voldemort does not have experience with. If he must spends his cognitive resources considering these aspects and cannot draw from experience it makes mistakes much more likely.