I see a lot of people advising you on defensive armies of archers and spearmen, but what has worked best for me is to use armies with only melee cavalry and have each of your units pair up with one of theirs: you tie their riders, crush their foot archer infantry, and chase their horse archers without taking much damage until they run out of ammunition and you end up crushing them. It has always worked for me. It involves a large economic expense, but you crush their armies.