Stabalise your econ and get some decenty units worked up. In the field, put your infantry in a deep formation, with support where needed by archers, cavalry, and other infantry. Keep some men to the side just a touch to the rear, to see if you can flank the enemy on the side or the behind (generally use cavalry/light infantry) If not, puch your cavalry into the enemy infantry, and immediately follow up with heavy infantry, you can usually break an enemy line in this way. Have archers fire upon the enemy in the rear ranks (like tehir general, if you kill him, they lose alot of morale). If you keep your formation in shape while breaking that of your enemy, you can snag yourself a phenomonal victory!

I would practise these techniques on the custom battle before bringing them to the campaign.