Haven't played it in a long time (so please forgive any inaccuracies) but ...
The AI is basically stupid. You can usually position your siege weapons and archers such that the AI either can't or simply won't touch them. At that point, it's not contest. If that gets boring (did for me) then you can always knock down the gate and use your knights to just run for the flag.
Fighting in the field is a little more challenging, but what I find works best is to place your archers out of range, then send a single archer inching ever closer to the enemy. At the first sign of movement, have him run back to the rest of your archers. More often then not, the enemy will turn and retreat once they start getting hit. Repeat ad naseum until boredom sets in.