In pre-sales on steam

[Sneaksie]

Try to set the difficlutty to low. Did you move your ambushing forces to help your right flank after they finish their work on the left?

BTW, Northern Crusades is much more flexible in terms of difficulty. Since it offers sandbox campaign (with main quest) in most cases you can choose which battles you fight and when, as well as you improve your army. If a battle looks too difficult, you can choose different course of action or go for more easy prey first.