1. Work out the number of troops on each side. Anything with more than one attack counts as [number of troops x number of attacks], so 2 troops for a 2 attack monster, 3 troops for a 3 attack monster, etc.
2. Add the number of troops from both sides, divide by 100, and round down: this is the total amount of rounds in the battle.
3. Each side rolls 2d6 and adds the following modifiers:
- +1 positioned on higher ground
- +2 fortified (e.g. stakes, ditches, etc.)
- +4 strongly fortified (e.g. constructed fortifications)
- -1 outnumbered
- -2 outnumbered more than 2:1
- -3 outnumbered more than 3:1
- +1 better quality troops
- +1 more than 10% missile troops
- +1 each 25% of troops mounted
- -1 mixed species army
- -2 surprised or ambushed
- +1 each previous battle round won
4. Highest score wins the round. The winner rolls [1d6 x difference between scores] and loses that many men from his side, 25% killed, 75% wounded. The loser just uses [difference between scores x 20] and loses that many men from his side, 50% killed, 50% wounded. The commander chooses where casualties are distributed.
5. Continue until all rounds are over, one side surrenders, or one side flees. If one side flees, the other may let them go and automatically capture all their wounded, or continue the fight, adding +2 bonus to the next combat round.
I may tweak this. For working out troop numbers, it might make more sense to base it on the number of hit dice rather than the number of attacks, perhaps incorporating the asterisk system from BECMI. You could also add on further sub-rules, forcing commanders to define some sort of "marching order" so the casualties get distributed for some units and not others, or perhaps allowing the commanders some way of selecting which of the enemy units lose men.
Gertrude the Grimy leads her army to try to lay siege to Bartholomew the Bastard's castle. Gertrude has 400 foot soldiers, 50 knights on horseback, and 100 archers. The knights have two attacks because of their magnificent war horses, so they count as 100. Total troops: 600.
Bartholomew the Bastard has 350 foot soldiers, 100 outriders on ordinary horses, and 75 crossbowmen mercenaries. Total troops: 525.
The total number of troops is 1125, so there will be 11 rounds.
Bartholomew has used his scouts well and knows Gertrude's line of approach: he prepares an ambush from higher ground. He also has higher quality troops and more than 10% of his troops have missile weapons. He has 2d6+1+1+1.
Gertrude's army is surprised, but they outnumber Bartholomew's, and 10% of her troops also have missile weapons. She has 2d6-2+1+1.
For round 1, Bartholomew rolls 3, giving a total of 6. Gertrude rolls 7, giving a total of 7. Bartholomew may have prepared an ambush, but clearly this does not perturb Gertrude's men; she rolls [1d6 x difference between scores] and gets a 5; she loses only 3 men wounded and 2 killed in the initial fight. Bartholomew uses [difference between scores x 20] and hence loses 20 of his men, 10 killed and 10 wounded. Clearly, both armies have used screens of skirmishers and Gertrude's make the best of the initial exchanges.
For round 2, Bartholomew rolls 8, giving a total of 11. Gertrude rolls 8, giving a total of 8. Now, clearly, the bulk of Bartholomew's army is swinging into action - maybe his crossbowmen are now letting fly: he rolls [1d6 x difference between scores] and gets 6 - he loses 18 men, 5 killed and 13 wounded. Gertrude's army, on the other hand, takes 60 casualties, 30 of whom are killed; perhaps Bartholomew's intelligent use of skirmishers has tricked Gertrude's men into over confidence.
And so on, for another 9 rounds.