Suggestion
How about you roll Knowledge (Strategy) or some related roll modified for aid another from other PCs that have similar knowledges and then assign level of damage to enemy.
Let's say
DC 10 minor skirmish or prevented obvious incursion;
DC 15 removed full squad or prevented hidden incursion
DC 20 disabled or re-routed one part of the army or saved number of civilians from harm
...
...
...
DC 40 removed major enemy spellcaster or protected our own shield from being dispelled
All this by some clever use of terrain or quick strike force in strategic locations
Then you simply describe how clever we are
I know this takes some from our own abilities, but for big battles it's hard to describe everything that might come handy especially since you cannot predict what can be important in some clever scheme we might come up with
And let us join major battle then...
In GURPS there are rules that govern major battles in which each character specifies how heroic he wants to be and then rolls two rolls one is survival the other is fame. If you take +2 to fame roll you take -2 to survival roll. And it's hard to pass without injury (you need to pass survival by 10 or more). This reflects the fact thta whole day of fighting has passed and even skilled characters will get wounded. And it's possible to be reckless and heroic and still wind up without fame and vice versa