I once took out the ENTIRE Mongol horde - the year they appeared, in the one province where they appeared (Khazar) - with a dozen longbows, a few chiv sgts, but mainly eleven units of billmen. The longbows weren't hurt much; they shot in the beginning of the battle (screened by the bills), then withdrew. The sgts helped at first and were cut kind of bad. I lost half the bills.

I did it with some primo billmen (silver shields but no iron; a 7 star general)... and a LOT of time... ambushing the Horde in a forest. The forest kept the arrows off of me, and made my bills lethal to horses. This forest was in a corner of the map so I made an L along the edges... when a mongol unit went after one arm of the L, the other arm pincered. It's like: a huge pile of men go into a room. The door closes, the lights go out. Much thumping and screaming is heard. Then the door slowly opens again. One near-dead guy crawls out. Over and over again.

Actually quite a few got away from each unit, which the AI would quickly recycle. I had killed the Horde general early on with my longbows, which helped them route quicker. His gazillion MWs and MHAs mainly just used up their arrows pretty uselessly (being in a forest) and left.

This battle took ungodly time and patience - maybe 12 hours long, the one battle. (How I feared a power outage or CTD.) I would 'recycle' tired bills to the very corner of the map so they could rest.

Near the end of the battle, the Horde did the damnedest thing... they stopped coming to where I was and instead bunched up in a place on the map, the entire 16 units of replacements, not moving, not firing, all in precisely the same spot such that you could hardly see the banners apart. I surrounded them with bills on all four sides, gave the attack order, and the carnage was absolute. They did this with their last 3 waves of replacements... and I did, too.

The end-battle tally was 4,285 killed, 2,858 captured, and 449 lost (~300 bills, the rest mainly chiv sgts). But when I got back to the strat map, it said 8,624 had been captured (including 33 nobles); I guess the difference was the routers who had nowhere to go, being as how the Horde had just appeared and had no land.

That was my battle to end all battles. And thank god I didn't CTD.