Ah, this brings back memories of one of my older forts, where I had just lost most of my elite marksquad and one of my best veterans to an elite orc sniper. Just then, a horde of treants, werewolves, and orcs showed up on the very top of the mountain to my west, and to the north and south of my fortress, located in the middle of the desert. I dispatched armies to hold off the units to the north and south, but I missed the army on the mountain because of the difference in z-levels. While observing the battles, I saw the enemy horde charging down the hill. Since all my squads were out, I had no choice but to send out the two champions I had left guarding the entrance. One of them, a female swordsmaster, reached the front of the enemy lines first and went into a martial trance. Limbs and blood started flying into the air. One z-level above the battle, I saw the head of an enemy elite hammerorc rise up for a second before returning to earth. The battlefield for tiles around was strewn with carnage and armor that had been knocked there by the force of my champion's swings. Near the end of the battle, she was so exhausted that she was about to perish under the weight of the survivors, but then my other champion, the oldest and most experienced of my warriors and one of the starting seven, arrived and saved her.
I've also found that locking the gates and then reopening them breaks enemy pathfinding, allowing you to send your troops out to pick off individual squads while a back-up watches the main gate just in case. I used this strategy to break a massive siege of giants, their mounts, and their associated auxiliary animals, with half my army of twenty surviving.
I think these kinds of situations and battles where all you can do is send out your best knights and hope for the best against overwhelming enemy hordes is why I play Dwarf Fortress. Either way, you're guaranteed Fun...