Greetings!
Ave, Caesar Tiberius!
Well, first off, one has to remember the enormous and absolute advantages that the Roman Legions--or their fantasy equivalents--have over their opponents. Now, what is the nature of these immense advantages and superiorities? To begin with, for these to have any kind of meaning, you must be prepared to enforce realistic principles of reality, concerning equipment, supplies, organization, combat skills, and psychology.
What does this mean? Well, it generally means that you cannot play the opposing forces with having perfect discipline, complete equipment and supplies, excellent fighting skills, and complete and thorough command and control. The Romans, however, had all of these things, and it is why they ruled an empire that lasted over a thousand years, and why their enemies ended up dead, in chains as slaves, or in the sands of the arena.
Some examples:
Enemy forces should be more effected by morale. The Romans have superior morale. The enemy will break and run when they suffer, say 25% or more casualties. The Romans won't break though, hardly ever. This also means that when one unit or another lets say gets hit by two or more fireballs. The Romans wouldn't break up their formation and run, but the enemy will. Imagine being doused by burning, flaming jelly. Be sure to play the enemy as having a weaker, more fragile morale than the Romans.
Enemy commanders make mistakes more often, and more severely. They don't have the ability to understand the nuances and the dangers of different Roman tactics. Provide the enemy forces with some healthy negatives in whatever skill checks you deem appropriate--like Knowledge (War) for example, to represent them *not* being a professional military force.
Command and Control: This goes along with the aforementioned, but is also different. The enemy units and formations DON'T do what the leaders and commanders WANT them to do. It's hard to explain, but enemy units and commanders just don't "See" what the Romans see, and they don't operate in good formation, they don't change formation instantly when ordered to do so, they don't move in the direction that they are ordered to go, or if they do, they don't do it fast enough, or coordinated enough to be effective when they do get to where they were supposed to be. For example, the commander wants them to move 100 yards to the left, and attack an enemy archer unit. The commander wants them to do it NOW--when they need to really attack and hit the archers hard. Instead, what happens?
The unit doesn't hear the order, or they don't understand it fully for five or ten minutes, then when they do move to attack, only 75% of them do so, because the other 25% just get strung out and don't get it. The 75% that do get going in the direction they are supposed to get charging, but along the way, they get wrapped up in fighting with some enemy Legionnaires that have moved to block them. Finally, a number of desperate minutes later, say 30-50% of the original force actually makes it to where the Roman archers are, and attack. But all during this time, the archers were firing at them, and now an element of Roman cavalry has charged into the enemy force, further disrupting their attack on the archers.
What is the result? The Roman archers suffer 5-15% casualties, but remain a disciplined and devastatingly effective force, which continue to rake the enemy with fire. The enemy forces that made the attack? Well, they suffered 40-60% casualties, and in the end, after doing some damage, they retreat in failure back to their original position.
You have to integrate the "Fog of War" fully into the battle, and be sure to include the screaming, the dying, the sheer terror of battle. These factors go into bringing confusion into the forces ability to operate efficiently, while these things are true with the Romans, it is the Romans discipline and professional training, and command and control, that minimises these realities and their effects, upon the Roman forces. The enemy isn't prepared for this, and as the battle continues, their forces' morale erodes, and they become less and less coordinated, as they head increasingly into being defeated by the Romans.
Next point--the Romans professionalism even reached the lower commanders, so that unlike their enemies, even when the Legate's commands were not delivered in time, or were not understood fully, the Roman commanders where trained and professional enought to be able to figure things out, improvise, and still make different attacks work effectively, even often guessing exactly what the Legate would want them to do, so the Roman operations would be successful even under the most confusing of situations. This kind of "Command Resiliency" isn't something that the enemy forces have either.
Supplies and Equipment: The Romans are fully equipped for everything in war. They have plenty of Scorpions and catapults, siege towers, rams, and so on. Plus, the Romans can dig trenches and make fortifications in mere hours or days. Much faster than the enemy forces. Give the Romans a few hours, and they should have effective fighting trenches dug that give them some protective cover, and traps that hinder enemy cavalry charges or raids into the Roman area. Overnight--or three days, and you should up the advantages for the Romans considerably. Also, the Romans are more innovative. They use stakes, pits, traps, burning oil, deception, all kinds of things in their prosecution of war. The Romans might also make a night attack against the enemy encampment, or they may make feint attacks, just to keep the enemy forces agitated until the real attack comes at dawn, from the entirely opposite direction, because while the Legate sent 10% of the Legion at one point to make spoiling raids throughout the night, he left another 10% at the encampment to move about and set all the watch-fires to make it look like the encampment was full and occupied, when in reality, the Legate has taken the remaining 80% of the Legion in a long night march in a great circle to approach the enemy encampment at dawn from the rear--where the Roamsn then pour into the enemy encampment with the element of surprise, and the enemy is immediately confused, and soon slaughtered, for example.
The enemy isn't going to have all the extra weapons and equipment and tools that the Romans will have. The enemy might run out of arrows, for example, or fresh water. If the Romans start a brushfire, and the smoke billowing through the area makes the enemy more thirsty, they drink more water, and deplete their water supplies at a faster rate. The Romans, however, have plenty of water, and will always have plenty of arrows, extra swords, javelins, armor, and so on.
Ok, these concepts must be integrated or else the Romans' superior training and discipline don't really mean much. These ideas are difficult to integrate as a "mechanic" per se, but that is where you, Tiberius, the DM, must take all of these details and factors into consideration. The enemy will simply not have all of these advantages, and it should be remembered that it isn't neccessarily *one* of these factors that are so decisive for the Romans, but it is a grand mixture of *all* of them working together, like a well-oiled machine. The Romans didn't do any of this by mere chance and wishful thinking. They *practiced* doing all of these things, under all kinds of weather, in all kinds of terrain, on 2 hours of sleep, no food for two days, while making the legionnaires march an extra four hours out of their way just to add to the pain and exhaustion. The enemies of Rome didn't train this way, and were never physically--and more importantly, *psychologically* equipped to fight war the way the Romans did. The Romans *MASTERED* the art of war.
So, remember, make sure you integrate all of these factors into any battle that the Romans are engaged in. As an added note, the Roman legion operates all as ONE--totally united, totally disciplined, totally committed, and totally trained--to engage in any battle. The enemy forces simply do not have this kind of esprit de corps.
As for the magical battle at hand, well, remember, even against undead, the Romans will know that such exists, and will have equipment, tactics, and training to respond to them effectively. Enemy wizards will be targetted by groups of hunter-killer groups of archers and rogues designed to target and swamp enemy wizards and clerics for death. Don't pull any punches, and this is where the Roman discipline can shine yet again. The Romans will stand there even in a vulnerable position in order to finish off the wizard or the cleric. Make the battlefield very dangerous for enemy wizards and clerics, because the Romans would make it a top priority to have the enemy wizards and clerics killed, while attempting to keep their own wizards and clerics alive and protected.
To give you another perspective, real-life Romans often fought battles where they were outnumbered two, three, or four to one--and the Romans routinely emerged victorious. Think about how awesomely effective one Roman legion would be in this battle, let alone two or three, as some are on the way as reinforcements. The enemy doesn't have much hope, even if they think they do. They had better pray to their gods, and pray well, for soon they will meet them. The Romans will send them there when battle is joined!
Semper Fidelis,
SHARK