[Manuk]

Lots of archers work like artillery. just place them in the back with some forces to hold the enemy. Fire arrows makes this more deadly also if you use fire magic.
Also if the computer is sieging a castle they will never abandon siege (for what ive seen). So if you capture all the surounding provinces and then attack their army with yours you will kill everyone once they rout (sieges take time, so you can prepare this).