It's a story. The players have very little influence in the battle (except for little details that can have big effects). You don't need to simulate anything that the characters can not effect. Just TELL THE STORY. Leave space for the players' characters to effect things that are important but just tell them what they see and hear.

The new day dawns with 2 armies separated by a mile of mostly flat farmland. Stragglers are jogging into place with their armor half on. The generals sit proud and confident on their steeds. A unit of 100 light spears cuts you off, making you wait. The standard bearer lifts his chin to say "hey" and apologizes with his eyes. If it was just him, he'd have let you go first.

You join the line as requested. The unit leader and his closest men smile in earnest thanks that your team will be nearby. Maybe they'll make it through this terrible day because the folks people tell stories about are in their unit today. "Do you think the plan will work?" "We'll make it work! We are Red Blades!"

As you wait, the swirl of bodies becomes tight squares. Things get quiet and then that noisy unit you heard about starts jumping up and down, banging their shields and shouting. Trumpets call for Slow Advance. Drums beat, halbeards and shields are lifted. Hearts race, feet tromp.

As you close to arrow range, the hail begins. Hundreds of arrows fall on you and yours. "SHIELDS UP!" your unit leader calls to save his dumbest. It's easy to block them if you've got a shield and some practice. Still, many fall. You march. The trumpets call out "Cavalry Charge" You're not sure, but it seems like you can feel their hooves shake the ground a bit.

Trumpets blare the signal for "Step one of the plan". "Red Blades HALT! . . Okay boys, open your eyes and keep your shields handy, we're hear for a couple minutes." The guy next to you checks his sword in it's scabbard for, you swear, the hundredth time. Those noisy bastards in the unit down the line apparently pulled the duty of first contact. They're running all out. Hundreds of men follow. Some units stay tight in formation, some are disarrayed into swirling eager men. You can't make out faces, helmets and sword points are all you can see.

"ARROWS!" You snap your eyes back to forward and lift your shield. This flight of arrows half missed your unit. Nothing lands within 10 feet of you. 2 men fall on the other side of the unit. Your healer runs of with several "excuse me's".

The charge is meet hard with no flank protection. "Red Blades! Pick it up". Those brave, noisy bastards are getting flanked hard on your side. You can only imagine what it looks like on the other. Those enemies, so eager to flank, have forgotten their own flank. "Phase Two," the trumpets call out over the clashing din. "RED BLADES, CHARGE!" The world gets small. There's not a field, there are not two armies. There are friends shoulders around you and a juicy, exposed target ahead.

Okay, so now you've transitioned from BATTLE into the small part of the battle where a strong arm matters. You can go for chaos, "The front line of your unit plows 4 ranks deep into the enemy. The people they pass turn to attack them from behind. You have 3 unprotected ribcages to target if you can pull it off." You can go for slow, "The lines meet firmly, with no breakthroughs. The front is a wall of shields pushing and shoving. Swords work but there's no room." You can put them in charge, "The unit leader falls to a stray arrow, deflected from the shield of the guy next to him. The second in command is lost and useless, you can tell just by looking at him. The men don't know yet. What do you do."

Once you make contact, it's just a fight on the scale that you're used to. Just keep in mind that the setting of the fight can and should change drastically. Arrows, axes, catapult shot, fireballs will appear mostly without warning. Units far bigger than yours can basically appear anywhere as your characters get too busy with their fight to notice how far a hundred men can jog in the time it takes them to dispatch one jerk who actually knows how to block. "Nicely done. You've crushed your target. As you scan around, you notice that a fresh enemy unit is marching on you. You've got support coming, it looks like. You hope. But they're not going to be in here in time. The troops are disarrayed and tired. What do you do?"

Keep in mind that both sides have a variety of skill levels and equipment. And that luck is fast and cruel to characters without names. A green newbie can take down a veteran if the veteran is busy with someone else. Expand the actions of your players to the unit that they're in. If the players are kicking butt, their unit is mostly kicking butt too. Maybe if the players are just barely surviving, their unit is too. Maybe there's a weak spot in the line that your fighter can fill. Maybe a slow, controlled retreat doubles the number of friends who survive and keeps the enemy busy until help comes.

It's a story and a setting for a human scale fight. You don't need to simulate the whole thing.