Keep your army together and put them in guard mode. I've noticed the A.I. with the Romans will send 1 unit at a time to try to open up gaps in your line. And whats the sense in countercharging a unit that is suicidaly charging your front line. Plus the A.I. strings out there units and I box mine so when they do there suicidal charge they are attacking 3 units at once causing a quick route do to morale penalties. If you counter charge though that will open gaps in your lines and they will start rushing there army at you.