The best way I've found to deal with the Horde stacks is to engage them in night battles in the field. Letting them siege you is dangerous if you can't beat them in a sally, since their multiple stacks can renew the siege before you can repair and refit even if you do beat them. Engaging them in the field without night battles is dangerous since they usually cluster so closely that you have to fight at least two stacks at a time. Thus, attack a night and eliminate on stack at a time.