If you said what faction you mostly play, i can make something of this.
But, i say do what Draigh tells you.
Here's something of me, i cant help not to help you: archers are useless against legions, except for the high end archers with upgrades. And as always a valid tactic, use the hammer-and-anvil (pin with infantry and flank with cavalry). If you command your own sort of legionaires, give the Romans something of their own shit, spam Pila.
You should try to break one cohort's morale in melee combat with fire weapons and flanking. Once they rout, flood like water through a broken dam. Send all your cavalry to charge through the gap followed by any reserve infantry.
To make this work out good you should pick the cohort which forms the center of the Roman battleline. Keep pounding that cohort with any missile units you have (yes even low end archers, throw everything you have at that cohort). Once in melee combat, they are broken rather fast, and than take advantage of the gap that is created.
SC
Dont be an Onager, use one instead.
"The Pope? How many divisions does he have?"- Joseph Stalin