Sort of related to the zipper and gunpowder tactics, killing the enemy is much more difficult than routing him. I started having much better battles when I stopped thinking in terms of destroying the enemy and started trying just to beat him. So use your missles, flaming arrows or gunpowder in the front and use horse archers to maneuver into the back, concentrating on one section of the line and then hit it hard. Always try and kill the general too, as this makes routing your foe much much easier.