Simple answer : you have to outplay them .

tips to improve your strat:
bring 1 unit of uirodusios
Add 1 unit of Gaesatae
You have cheap AP units as mercs in Appea Gaedutos. they can be both anti cav / flankers
use belgae swordsmen/belgae cav ..they're much better

if you manage to win the cav battle you got a shot ..you have a lot of scary units , their cumulative effect should give you that