It's all about maneuvering your units.

I had one battle against the Mongols in which I got them to give up the higher ground. Along one edge of the map was the hill with it getting steeper at the corner (to my right). I initially moved my units around to their left to gain some better positioning, and they began to move towards the corner and setup their defensive line. I moved my units forward along the side of the Hill, and eventually their units rather than holding their ground started back down the hill but still along the other side of the map. Eventually the high ground was clear, and eventually rushed my units the last bit up the hill and reestablished my lines. Once my archers began to start firing, is when they decided to attack me with them now coming up the hill. I charged my units down the hill to gain the advantage.

I've done this a few times. In another battle in which I had some Turcopoles I managed to get the entire enemy army to flee after I had used my Turcopoles to lure some of their units out of formation and with doing the HA dance, the AI decided they didn't want to deal with further casualties, and decided to turn and leave.

Then there was another battle in one of my earliest campaigns where I was fighting on a mostly flat wooded region. I setup my lines in loose formation and my armies must have looked larger than they wanted to deal with, and decided to flee without a single sword swung or arrow fired.