Dungeons Unleashed Battle System
The Dungeons Unleashed battle system has been playtested extensively in my "home" Crimson Shield campaign and in a number of games at conventions and events. You can grab a copy of Dungeons Unleashed here: https://composedreamgames.com/marketplace/dungeon-unleashed?search=unleashed
Desired Feel:
A) Units fighting each other should feel like a back and forth, with one unit seemingly having the advantage, then the other. 
Unit battles should be short, ideally a few roles in most cases.
C) Troops within a unit are injured and killed, but this isn't key to success.
D) Players own characters should be able to influence the unit, and also get into personal battles.
C) means the main "damage" stat is morale/unit cohesion. This represents: staying in formation, not being encircled or split, courage and experience, in other words does the unit keep fighting or retreat or surrender.
To allow for A) units can also gain morale. This includes PC's committing themselves directly to the fray.
To keep things short, means morale has to stay low. I think the highest value I have ever used is 4.
A unit engaging and then immediately breaking off is pretty dull, so 0 morale doesn't mean that a unit immediately breaks, rather they are at a tipping point, and a leader or PC could do something to keep them engaged for at least one more round.
Basic die system:
D12s opposed dice pool.
Any dice that are 4 or below are dropped. These aren't considered basic successes.
Dice above 5 are compared to the opposing units dice, highest against highest and so on.
I think I've given one unit 4 "fight" dice once. That's a lot, even 3 dice is high/rare. Often that third die is for a temporary tactical advantage like a mounted unit charging at a infantry unit.
Units have a morale/cohesion stat, and a fighting stat. I usually eyeball the relative strength of a unit and make adjustments to these. Fighting is how many dice they roll, morale/cohesion is how much they can withstand before dissolving/retreating.
20 elite heavy infantry against 200 raw conscripts on an open plain would have their fight dice, and morale reduced, while the raw conscripts would have more. 30 human infantry against 5 minotaurs might be considered fairly even.
Some examples units:
Raw conscripts: fight 1, morale 1
Experienced levies: fight 1, morale 2
Light infantry / skirmishers: 2 / 2
Longbowman behind stakes: 2 / 3
Heavy infantry: 2 / 3
Elite heavy infantry: 3 / 4
Cavalry: 3 / 2 but usually 2/2 against other cavalry
Charging minotaurs: 4 / 3
The roll examples:
In Simple Superheroes and in Dungeons Unleashed rolls between opponents are opposed and only the attacker deals damage. In battle however, "damage" to both sides is resolved in a single roll. Any basic success means troops are being injured or killed. Of those dice, any winning die damages morale.
Let's say a cavalry unit has engaged an elite heavy infantry unit:
Infantry: 11, 5, 2
Cavalry: 9, 9, 5
Infantry has two basic successes, this means they are killing cavalry. Scale this to the number of people in the unit - so if there is are 300 maybe 20 cavalry have fallen. If there's only 20 perhaps only a couple have been taken out.
Death/injury is mostly color, though it can inform post battle activity and outcomes.
More important is affect on morale/cohesion. This is determined by comparing those rolls that are basic successes.
Infantry's 11 beats the Cavalry's first 9, the Cavalry's second 9 beats the infantry's 5, and the Cavalry's 5 beats the infantry's nothing.
So the cavalry lose one morale, and infantry lose 2. Leaving the cavalry at 1 and infantry with 2.
The cavalry strike a heavy blow, the infantry buckles but doesn't break, and the cavalry also suffers casualties loosing some of its cohesion.
Second roll
infantry 8, 3,1
Cavalry 8, 4, 4
The 8s are tied and ties normally go to the defender, in the unleashed engine, but here they just indicate a stalemate. They are still successes so an equal number of men fall on both sides. But no shift in cohesion occurs.
Third roll
Infantry: 12, 10, 7
Cavalry: 10, 7, 5
A clean sweep, the cavalry drops more infantrymen but their cohesion is shattered by the infantry's attacks, suffering 3 "damage" to there morale, which puts them at negative 2. It will be pretty hard for anyone to rally the shattered cavalry into anything that is able to contribute further in the battle.
On Rallying, Raising morale and what your PCs do:
Rule 1: Always have at least 2 units for both sides.
Rule 2: PC chooses/decide what unit to stand with.
Rule 3: potentially allow a PC to raise a units morale with a suitable talent(skill) roll or give a one time boost to units fight. This can occur before or during the fighting, but once per PC.
Twork fight boosts shouldn't stack, rather they should be used on different rounds.
Rule 4: allow a PC to commit themselves to stop an opponent from doing the same for their units (an enemy shaman ensorcels enhances all his front lines weapons making them glow green, your PC wizard dispels it) change the fight to PC vs npc at this point. Unit combat continues, but that PC can't rally or do 3 or 4 until their foe is dropped.
Block or restore 1 morale damage:
A PC can commit themselves by charging directly into the fray to stop a point of morale damage.
This comes with personal peril.
Narratively this is usually a PC seeing a particularly dangerous or successful foe and engaging them: an ogre, a trio of heavyly armed knights, the enemy centurion. I often give two pc actions per unit action, in these situations.
A reserve unit entering the fray does the same (block or restore 1 morale) depending on size it could attack independently as well.
A retreating unit can be rallied, this usually requires some leadership action from a PC or allied NPC, and some some time to return. Rallied units are fragile usually only having 1 morale unless they incorporate some members from a more experienced unit.
Units at 0 morale are on a knife's edge, they might retreat in an orderly fashion, keep fighting or be on the cusp of being surrounded. I make a judgment call here based on how the opposing unit(s) are doing and if they dealt morale damage to the opponent on the last roll. Two units at 0, will likely disengage and return towards their side of the battlefield. In relatively good shape to reengage in a later clash if the overall battle allows.
The troops in such a unit feel they have "already done there bit," so they aren't eager to re-engage.
Comments
Playtest’s run with basically this engine
These are all from the “Crimson Shield” mercenary company campaign.
Crimson Shield vs pod-men (session 1)
sortof some proto-type style things were rolled.
Crimson Shield vs The 30 of Terrathor Bridge (session 4) https://composedreamgames.com/forum/discussion/comment/74/#Comment_74
Units:
Crimson Shield heavy infantry
Crimson Shield skirmishers
Halfling slingers
vs
Undead elite heavy infantry
Undead elite skirmishers
https://composedreamgames.com/forum/discussion/comment/77/#Comment_77
Crimson Shield infantry
vs
Lizardfolk raiders
most of the pc’s engaged Apraxis the shaman, and his bodyguards directly. So I broke a “rule” and had only one unit vs another.
https://composedreamgames.com/forum/discussion/comment/79/#Comment_79
Lizardfolk
Crimson Shield
vs
Wight+elites
Undead warriors
Tanagreal the green dragon
https://composedreamgames.com/forum/discussion/comment/95/#Comment_95
Dogmen skirmishers
Pigmen
Wolfmen
vs
Crimson Shield infantry
Crimson “Hammer” cavalry
same link
crimson shield
Cecilia (necromancer + undead guards)
vs
dogmen (kobold) archers
pigmen (orc) “heavy” infantry
wolfmen “calvary”
n.b. small skirmish so Bulch acted independently against one wing of the dogmen, and disrupted the wolfman sneak attack on the necromancer.
or The Defeat of the Beastmen of Kadesh