The enemy armies are way too powerful, but there is a group of local mercenaries (or perhaps an underground resistance) in the area that might be persuaded to aid you if you can meet a price and/or supply a need (which might involve a raid on the enemy to get a valuable asset).

Hold a Key Position: If successful, the enemy will have to go around and confront regular grunts on a more solid ground. So, your success means the enemy is more likely to lose.

Slay the enemies magical support. They only have one decent mage or cleric to support a huge army and your guys know where he is and how to get in striking distance. Slay that enemy and their troops will lack a key element of support.

Steal a Shipment of Artillery (i.e. wands or scrolls): The enemy has a bunch of low-level mages who will be supplemented by wands or scrolls (from a high level mage well back to the rear) to enhance their ability to throw fireballs, etc. into your side. The shipment is coming in and if your characters get it, then the asset not only doesn't get used against you, your side ends up using it instead.

If that damn bursts, it floods the enemy encampment.

Poison the well: I mean the cavalry to save your city is 8 days away from the city your defending, but the enemy is 5 days away unless you can destroy that one well along the way. Then they'll have to go around, taking 3-5 extra days, now that just may be enough time to get your re-enforcements.

Okay, the enemy camp is pretty well beyond your capacity to take or destroy, but you can escape into the woods pretty easily. If you can do enough to damage on small raids, you can undermine morale and/or get them to move camp.

...better yet, if you can draw the enemy after you as a decoy action, then perhaps the real attack will have a chance of success, but the enemy has to think you're the real army.

The second in command of one major army among your enemies is known to be against the war., If you can kill the main guy, his second will surely pull out of the conflict.