I tend to take down Roman armies that have a chance of fleeing by deploying cavalry as far forward as possible, then sending the cavalry at an allout gallop at them. If they start withdrawing then the cavalry will be able to rout at least a few units of infantry. If they don't, well, wait for the infantry to come up and pursue conventional battle. Always worked for me. Some battles ended with only the enemy general bodyguard making it off the map.