I almost wet my pants! You're taking on the faction leader and he doesn't have any missile troops?

Take your army and fortify on a good defensive position. Then take your mounted X-bows and pin-cushion the faction leader. If possible try to goad his knights into your kill zone and use your archers on those horsies! If you do this right (and take your time) you should suffer ~5% of the total casualties and make them go rebel.

I love archers!!!