Chariots

[Sarcasm]

That's about it. Try to mass them (use more than one chariot unit on top of another) and use them in 3 or 4 rows. Shallow formations are espeacially vulnerable to chariots. Stay the hell away from javelin throwers (kind of difficult against other celtic armies, I know).

Use some skirmishers, in open formation, to thin out their ranks and so they waste their javelins on cheap and difficult to hit infantry instead of your valuable chariots.

Then charge through a group of heavy infantry and follow up with some of your own. They'll be extremely demoralised and out of formation and should break soon afterwards, or be at a great disadvantage throughout the remaining fight.

You may want to wheel around and hit other parts of the line where the fighting is the thickest to unbalance things but pull out immediately.

Oh by the way, enemy cavalry just go down like flies against your chariots...just keep your general's flank away from serious fighting.