The Grim Tales Mass Combat PDF is really great here. I'm planning to use it a few sessions from now. I hope it goes well. I don't know if you can get the PDF anywhere legal anymore, but I know you can get the
hard cover still.
While the PDF has lots of rules for cool things like Charges, Formations, Holds, Retreats and so on, the basic concept can be boiled down to this: EL attacks EL. That's it. You're done, you don't need more of a system. Although I'll provide an explanation.
Go to the DMG, page 49. Extend table 3-1 using this concept: any time you double the number of combatants in a unit, their EL is +2. So let's say you want a unit of 20 1st-level fighters. Each fighter is a CR of 1. The table doesn't have info for anything beyond 12 enemies at once. So to get to 20 fighting men, we'll use the table to figure out what the EL of 10 men will be, then add the +2 to the EL when we double the men to 20. According to the table, 10 fighters of CR 1 have an EL of 8. Double the 10 fighters to 20 and we add +2 to the EL for a total of 10 EL.
OK? Now you can figure out the EL for all your combat units. Let's say you have 10 units just like the EL 10 unit we just built. And the other side has the same. Now you pit them against each other. Each unit engages an enemy unit.
Each unit on each side rolls a d20 and adds their EL. If a roll beats 10 + the enemy unit's EL, then they reduce the opposing unit by 2 EL (halves the size of the unit due to death, desertion, or debilitation). Because you resolve both sides at the same time, it's possible for each side to score against the other and simultaneously reduce each other by 2 EL. You keep doing this until the combat is over due to surrender or death.
When you roll the dice, it does not represent round-by-round combat. Instead it represents many rounds. So if a unit of 20 fighters lost 2 EL due to a bad combat roll, it's likely that what happened on the battlefield was a brutal full minute of warfare that resulted in a lotta dead & wounded. If both sides lost half their troops, well, victory comes at a high cost.
EL is a very handy abstraction because it summarizes the effective armor/weaponry/class/monster powers, without requiring you to painstakingly stat it all out. It essentially uses existing D&D rules, and it's a very fast & easy way to resolve combat among hundreds of combatants.
The rules have a lot of extra cool stuff for having "heroes" (your players' characters) lead units, along with cool ideas such as having dragons or other magical monsters involved (how do their breath weapons affect entire units, for example). But you mostly don't need the details. You can make something up.
I wish they'd republish the PDF, though.