Whom do you really want to win this fight? Yes, a gamemaster is supposed to be neutral, but you know how you want the adventure to go, so make it work out. Simply describe the other fights in glowing terms, with much hewing off of heads and sundered limbs, while running the player-character battles normally.Take note of the sentence I've bolded above. I highly doubt that this is the first time that this sort of sentiment appeared in a RPG product. Indeed, I suspect that some enterprising person could probably cite an example of a similar sentence in a product from the first five years of the hobby. Likewise, I think it's pretty clear that advice presented in The Shattered Isle is not intended to be a manifesto of a new way of roleplaying. It's just reassurance for the referee to trust his instincts in ensuring that things "work out" as they should.
Thus, if the player-characters are losing when you want them to win, simply describe how their allies are defeating their foes with ease, then turning to assist the adventurers.
Similarly, if you want to drive off the adventurers and they are doing well in their own fights, describe the steady attrition of the allies until they find themselves fighting multiple foes, by which time the intelligent adventurers should try to get out of the mess they're in by voting with their feet.
But make no mistake: this does represent a shift in the conception of RPGs and the role of the referee in RPGs and not one for which I have much liking these days. I think it lends more credence to the notion that the mid- to late 1980s are when certain changes in the hobby became not only widespread -- they were probably popular long before this point -- but when gaming companies began not only to acknowledge but to encourage them.