Hard to give you specific advice as its all a bit situational - and depends a bit on what you have to hand.
At the end of a battle, cavalry are a real force mulitpiier, not much use with the Romans in the early scenarios but helps in the later games. In effect if you win, and have cavalry, you will inflict a lot more hits.
Numbers count, while the combat routine makes the enemy more likely to try and pull back if badly outnumbered, every now and then they will stand, at that stage raw numbers can really convert a small to a big win.
Ranged fire helps - again less for the Romans in the BoR scenarios, but this will disrupt before combat making any melee victories easier and more dangerous.
Operationally, try to take some MC in the likely retreat province. A small force of cavalry or light infantry in the red-green stance can close off retreat routes or at least dictate the region of retreat. This is very useful in BoR due to the small scale
If you attack, try not to do so with low cohesion. Even if you win you'll find your forces break off early.
Leadership traits can all come into play, some commanders have traits that build on the concepts above such as more troops/frontage, pursuit, speed (ie less cohesion loss as you move) etc.
Your opponent matters. If you are fighting mostly light troops in poor terrain they will tend to escape. Think of it as less of a stand up fight with formed battlelines and more a running skirmish. Welcome to the realistic AGEOD problem of how to deal with irregulars/rebels and poor terrain. In the games time frame this explains why the Jewish revolt lasted so long and why the conquest of Spain was over centuries. Boxing them in by getting control of the wider region is your best strategy.
and welcome to the forum