How about some rock-scissors-paper?
You could have your men "raid" and only engage a portion of enemy troops. If, however, there is an ambush, the "raid" backfires with severe penalties. If there is no "raid" and there is "ambush", the "ambush" backfires.
Then have some randomness in it too, perhaps dependent on leadership skill. Both raids and ambushers would fail less frequently with high leadership skill and low amount of men to command.
A "raid" that succeeds hits a part of the defenders for some rounds and then the troops start traveling back to their original region. A "raid" that fails makes it a normal battle with the raiders on the front lines (further out than normal, perhaps).
So, to the essential feature would be "raiding", which would enable a smaller force to take a bite on the larger one, and "ambush" would be to counter that.
