I got to command the barbarians, I started off sending my entire cavalry to a face off against their roman counterparts.
Half of my horses stood to face the roman cavalry, and the other half wheeled to charge at 2 cohorts of auxilia nearby, which then reformed into square, but suffered heavy casualties nontheless.
In the middle of the field, the romans advanced, supported by their intimidating artillery, and my warbands endured quite a punishment from those well-drilled legionary cohorts.
After several clashes, the roman cavalry finally mamanged to put most of my horses to flight, and the single remaining unit I had (which was harassing their auxiliary infantry squares) was completely surrounded.
Meanwhile, my warbands were also having a hard time holding their ground against the roman onslaught. The enemy pushed forward like an unrelenting tide of steel, breaking several of my warbands and spreading fear and disorder on my ranks.
The remains of the gallic cavalry, surrounded and cut down to the last man!
My gauls fought bravely, but poor command rolls kept me from getting advantageous charges, so instead my formations got assaulted over and over again, and my divisions began to fall apart.
When the celt left flank, collapsed, with their cavalry completely destroyed, the day was won for the romans. The sight of thousands of roman horses moving to flank the gaul centre was enough to put all remaining warbands to flight in despair. Many were cut down by the pursuing roman horses, ending the battle in an utter desaster for the gauls.