So, I was playing as Koinon Hellenon (the classical Greek city-states, allied together to resist Macedonian and Epirote invasions), and had managed to, at game start, beat off an Epirote army and get ceasefires with both starting enemies. Then, I allied the Epirotes (they're desperate for allies, as they're at war with Rome, Carthage, and Macedonia, all of which are stronger than them). I entered a battle with a full stack of my own, dragging two nearly-full stacks of enemies and Pyrrhus Aetos' (the Epirote King and their only competent general) stack into the battle. I then proceed to sit on the sidelines as Pyrrhus hilariously owns the first enemy army, before getting himself killed in a remarkably stupid way. The first enemy army routs, Pyrrhus' remaining forces rout, and I defeat what's left of the second enemy army for a grand total of ~4500 enemies killed for 250 casualties of my own. Also, I killed 1/3 or so of Pyrrhus' stack, by virtue of just not entering the fighting until he was dead. Epirus is now greatly weakened, the rebel city is mine, and I have suddenly become the strongest power in Greece (I mean, I was before, by virtue of having just enough soldiers to defeat the AI armies with superior tactics, but now I'm actually probably the strongest power in Greece).
Europa Barbarorum II; a mod for Medieval 2: Total War.