Treat each unit as a character. Average the HD out.
For outnumbering, treat the more numerous side as having +1 HD. For every multiple by which you outnumber them, increase the unit's HD. So a goblin warband that outnumbers their foes 2:1 but not 3:1 will be treated as HD 3 - 1 creatures rather than the usual HD 1 - 1.
When numbers change via casualties, adjust HD from outnumbering.
The percentage of hp lost equals the percentage of units lost, rounded down.
A side that's defending a fortification suffers 1/2 damage. Make other ad hoc adjustments as it seems good to you.
Roll hp, initiative, and morale as normal.
Then have each unit attack as normal.*
Done.
*If one side has a special power, like a medusa's petrification, adjudicate that as you see fit. This is supposed to be quick.
