Book 1 mentions that Orcs make a habit of running away. I'm of the mind that most intelligent creatures will disengage from a fight when the outcome seems obvious. Most peasants don't understand, so I figure a 10th rank sorceror could hold off 300 peasants by using one FIRESTORM spell, and threatening to send the rest of them the same way.
I think I recall reading sources that suggest that ancient or medieval battles were resolved quickly when one side started to rout. Perhaps some formulas could be applied
50% casualties
1 orc leader equals 5 orcs
Players encounter
1 orc leader and 9 orcs
Orcs may rout upon defeat of either
1 leader and 2 orcs OR 7 orcs.
Not all at once, but when 1 more orc routs the others will have to face some kind of penalized morale check.
As opposed to a party of Ta'ashim fanatic Assassins who never rout (0%) whilst intoxicated with hashish, or 80% rout for drunken thuland barbarians? Maybe we can create guidelines.