I found the best solution to the roman 'quality' troops is to insure that your troops are experienced. Light chariots and swordsmen are what you need and try to suck other units into fighting your infantry in snowy terrain or wooded areas.
I found that if all my units are silver exp, I can crush practically anything in my path. When other units moral breaks and they run, make sure to exterminate as many units as you can before they flee off the edge of the map and don't let them regroup. If the romans are producing new, green, troops all the time rather than retraining the ones you defeat, they don't have a chance.
Hope that helps.