This is meant to be a simple, yet strategic way to deal with mass combat situations. The main mechanic of the system was actually created by my highschool roommate.
First, you seperate the large numbers of creatures into groups of size n, such that n >= 10. These groups must be made out of "identical" creatures. It is pretty obvious what this means. This group will from now on be considered a "unit". This unit is essentially now a single creature, with all of the same stats and scores that the constituent creatures had in common. This means THAC0, number and type of attacks, saving throws, etc. There are, however, 2 notable exceptions: hit points, and damage.
Instead of the standard hit point and damage rules, we will switch to a "hit" system. This is essentially (hit points)/4. So, if the average creature in a unit has 40 hit points, than the unit has 10 hits. This is also done with attack damage. For each unit attack, the average damage is calculated, divided by four, than rounded to the nearest whole number. So, lets say you have a unit of longswordsman. The average damage is longsword average (4.5) plus two for specialization. 4.5 + 2 = 6.5. Divided by four, you get ~1.6, which would round to 2 hits. So, the longswordsman unit would do 2 hits per attack, and have 3/2 attacks (specialized).
After these easy calculations, you can do the battle, treating each unit as an individual. So, each one can move independently, and attack independently. They have THAC0s, and ACs. So, you use the same AD&D combat system, but with hits instead of hit points.
A unit doesn't die until all of its hits are depleted. At this point, the unit is effectively disbanded. This means that 90% of the constituents are killed, and the other 10% are either wounded or running. Effectively, the unit is out of the combat.
Now, there are only three more things that need saying.
In any battle, one of the major problems is communication, and command of the troops. This is taken care of through the simple mechanic of "command radius".
Every creature has a command radius of 0 except for warriors. They have a command radius of (level + reaction adjustment[charaisma] - 10)/5, rounded down (can't go below 0, obviously). A creature with a command radius of 0 can always command the unit in which it currently resides. Command radii greater than zero indicate an ability to command units beyond just the unit in which the commander resides. Each command radius point allows the commander to direct the actions of units one hex away. So, a fighter with a command radius of 2 can command the unit he is in, plus any units within 2 hexes of him.
Normally, commanders can't communicate with each other during battles, so they can't coordinate attacks or strategize on the fly. This is because it is loud, and they are seperated by large distances. However, commanders who have crossing command radii can effectively communicate with each other.
This brings us to addendum 2...
In standard rules, hexes are 10' wide. However, for mass combat, this won't work, since units can be quite large. So, hexes need to be resized depending on what sized units you have. This is a relatively straightforward problem. Just assume that one person requires a 5' hex. That means 4 in a 10' hex, 16 in a 20' hex... So, you can get the following rough values:
|Unit Size||Hex Size|
And so on...
The last complication involves things that do area damage. Again, this is easy to calculate. In cases in which the area damages can effect whole units, just roll the damage, divide by four, and any unit within the area takes that many hits.
The only little snag comes when you are dealing with area effects that are smaller than a hex. For this, you just need to scale the damage by the ratio of the damage area to the hex area. This involves a little bit of calculation, but nothing that an 8th grade graduate couldn't handle.
An example. We are dealing with n=100 units, so the hexes are 60' = 3600 square feet. A 10th level mage fireballs a unit. Fireball is 20' radius, which is 20*20*3.14 ~ 1200 square feet. This is ~1/3 of the hex area, so you just scale the damage down by three. He rolls, and gets a nice shot of 48. Divide this by four, and then by 3, and you get 4 hits. The unit would then make a saving throw to possibly halve it. Simple.