Two moments of ownage tonight:
The first case was when the Gauls broke their cease-fire agreement with me, attacking a small army led by an untested general with several warband units. I had the general, three units of hastati, and a unit of light cavalry. They outnumbered me 2-1, fair odds when one army is a disciplined legion and the other is a motley horde.
Now, they were uphill and my cavalry would be foolish to take on a bunch of spear-wielders head-on. So instead I had the hastati (with Fire at Will turned on) hide in a stand of trees and sent my general out to taunt the barbarians in. They came charging down the hill as the general casually sauntered behind my lines, and only saw my infantry at the last moment. By then, however, they had too much momentum built up and ran directly into several volleys of javelins. This broke them just enough that, instead of charging in to melee and using their numbers to gain an advantage, they sort of crept up one by one, apparently shaken by the sudden casualties. As each warband began to rout, the general and cavalry chased them down. No survivors.
The second instance was when I wished to take a town that an army of Britannians were currently in the process of besieging. I couldn't assault the town while the Britannians were doing so, and the town blocked access to their army, so I had to go around a small mountain. When I got to the other side, guess who was waiting for me? A Gaul warlord. He immediately attacked, and the Brittanians got pulled in as enemy reinforcements. Total army strength was about 400 on my side versus well over a thousand.
Nowhere to hide this time, just some really nasty hills which prevented any one side from gaining and holding the high ground. The Gauls, being closest, attacked first. They somehow had no warbands of spear-wielders, but a couple of cavalry units, swordsmen, skirmishers, and of course the warlord. While I smashed most of them to pieces (killing the warlord) and many fled, they kept coming back to harass my army throughout the battle. Then the Britannians finally joined the fray with an army of just over 800. These guys fought like demons, not helped by the fact that my archers suffered a terminal case of stupidity and bad luck, losing over half their number while warbands chased my general around. Eventually we prevailed however, and they too started to flee. We couldn't chase them all down and lost over 100 brave Romans, but the battle was solidly in my favor.