I finally figured out how to win this battle. All you have to do is stand your ground...Since you have topographical advantage, it's practically easy to slaughter Napoleon's army as they march upwards. When a cavalry charge is imminent, wait for the last minute and switch your men to square formation. Following this strategy, you could kill 70% of Napoleon's men in just about 15 minutes. Just about time for the Prussians to join the fun and finish it.