This is something I learned from a Rome 2 gamer in youtube. It would require that you are fast at managing units in a battle. I recommend assigning numbers for each unit type so that it is a little easier to manage the units:
If in your campaign you are going to have an open field battle with flat terrain and even if the enemy has some numerical advantage but has no cavalry unit, you can use a special tactic specially if you have enough slingers or archers ( 6 as a minimum ) and some cavalry units ( at least 4. But preferably from medium to heavy cavalry ). This tactic would work specially if your melee infantry units and your slingers and archers can outrun the enemy melee infantry units.
First, avoid engaging your melee infantry units in combat. The first ones who should do the job should be your archers or slingers. Just make all your melee infantry units run away from the enemy melee units. And you can also make your range units run away from the enemy melee infantry units to different directions so that you have the chance for shooting the enemy units at their back or flanks as they chase them. You can use your cavalry units for distracting some enemy infantry units who are chasing your slingers or archers but this would require that you are very good at micromanaging units in a battle because otherwise your cavalry units could get caught by the enemy infantry units. At the same time you can take advantage of the moments when the enemy range units are unprotected to finish them all with your cavalry units but be very careful so that they don't get caught by some melee infantry units who come to rescue the enemy range units. Once you use up all your range units' ammunition, try to isolate each enemy melee infantry unit in order to charge at tha unit with your cavalry units from different directions at the same time. A melee infantry unit that is fatigued from running tend to rout when being charged by several cavalry units from different directions simultaneously. Never charge at an infantry unit only from the front even if you use 3 or 4 cavalry units at the same time.
The battle then should become easy. Then you finish off the rest of the enemy infanttry units with your melee infantry units which would be fresh.
EDIT:
The battle tactic I described above might still apply if the enemy has a few cavalry units ( like 1 or 2 or 3 ) in case you have enough cavalry units, engage them against the enemy cavalry units only if your cavalry units are heavy or experienced. If you can, send a few of your melee infantry units to help your cavalry finish off the enemy cavalry faster before the enemy infantry units arrive ( Many times the AI tend to first send their cavalry units to combat in case it is agressive )
In case your range units are all mounted archers, then it is much better for the tactic because you can safely make your cavalry archers fire at the enemy infantry units without the risk of being reached and caught