In the beginning don’t expand (hardly at all) so you give the A.I a chance to build up its forces so you will have a decent enemy to fight rather then stacks of 2 or 3 unties.
When out numbered form your men into a defensive square and prey that your enemy will get tired and rout before your men crack and run.
Don't move your archers behind the enemies back line as this is not realistic, all you are doing is exploiting the AI's own stupidity.
Build realistic legions and not legions of pure Pretorians and Urban cohorts.
1x General
2x Cavalry
2x Archers
2x Javelins Aux
10x Legions
2x Auxillia