You could do a point system.
Each would be worth one point. Each gets another point added if they can counter a unit in the enemy stack.
For example: Say there's a force of
4 Infantry, 3 Archers, and 3 Artillery
versus
2 Infantry, 5 Archer, and 3 Artillery.
Right off the bat: The battle is 50/50.
However, Infantry beats Artillery. Since StackA has 4 Infantry and the enemy has 3 artillery, StackA gets +3 points. Likewise, StackB has 2 Infantry and StackA has 3 Artillery so that's +2 points for stack B.
Current Tally: 13-12
Final Tally: 10+3+2+3 versus 10+2+4+3
Or
18 versus 19.