The AI...helped me? O_o
Just had a battle against two Boii armies. My full army, around 3500 soldiers and my ally's army of similar size. When the battle started, my ally sent a detachment to guard my right flank(and they did a good job O_O ) while the rest of the army tackled the second gallic army. They held their ground in formation and did not break until the chariots tore 'em a new one.
So...yeah, I need a drink.