So, far I've only played Roman factions my army composition has stabilized@

1xRoman General
plus two legions each of:

4xLegion Cohorts
1xArcher
1xLight Auxilia
1xRoman Cavalry
1xOnager

Standard deployment is three lines:

1st line Missiles
2nd Infantry + Cavalry to Flanks
3rd General & artillery

If attacking I open with a barrage of flaming Onager ammunition, then close to within archer range and take out as many as possible.

Finally, the lights and heavies go in with javelins and pilums to finish the job. I always have my heavy infantry on FAW.

The cavalry finish the job cutting down the routing units and ensuring only a handful escape. I sometimes add exotic units to the mix such as mercenary elephants in my north african armies though these have proved to be a mixed blessing when they go berserk and I lost one battle because they smashed through my own troops.