I know how to do that one because I was stuck on it for ages to! I found the easiest way to do it was to use the army that starts right next to the enemy to attack them (dont worry you will probably lose this fight but your reinforcements will rout them easily) and run your taisho up behind your reinforcements. The hardest bit now is to take out the cavalry on the hill. To do this you just use the guys in the forest (make them all attack different units) and then it will just become a small rabble which will in time rout the enemy. This worked for me first time I tried it and I hope it works for you to.