I assume that we're talking wooden walls here. Multiple breach points, you have eight foot units, they have seven units total.

That means that they can at best plug seven holes in their walls. So put eight (or more if you feel like going over the top) holes in their walls, and look at what hole they leave uncovered. Move that unit in, and then watch as they move their units to meet it, leaving more holes uncovered, move those units in etc. By the end you should be able to get a little maneuvering going, until you can hit one enemy unit in the back.