If the AI has an unbalanced army, you can attack with an army of counter units which is numerically smaller than the defensive army. The machine will make it's calculations based on pure numbers, and you can cut right through them, slaughter the prisoners, and deny the enemy the chance to consolidate its forces after a retreat.

I'm sure that taking advantage of the logic (or lack thereof) the AI uses to decide whether to stand and fight is an exploit, but it is an option.