I have found that a combined arms approach the best way for defeating the enemy. Catapults and ballista for long range death, bowen and slingers for medium range death and the trusty infantry for the close in work. With an effective spy network you should always know what your army is up against and can customize your army prior to contact for the best results. I tend to use small well trained, well lead, mobile army's supplemented by the local garrisons. It is necessary to micro manage both you ballista and your bowen but that is part of the fun......