An Update
So it happened last night. All my questions (or most of them, anyway), were for naught.
The dragon burst free of the wall of thorns and hollered for our blood. We heard the scream, some 800 feet away and through our newly constructed Wall of Stone, and decided to hightail it out of there. The rogue/monk was nowhere to be found (player is out of town and didn't leave his character sheet with the DM, grr snarl), but we decided we'd deal with his mysterious disappearance later.
Our cleric has been setting up shop in an abandoned dwarven temple not too far away, and has discovered that he can control the temple's guardian, a massive stone hammer-wielding dwarf statue that we thought was an elemental. So we headed back there to make our stand, buffing ourselves and summoning allies.
Summoning allies: what a dumb idea. I completely forgot about a dragon's fear aura. My black bear took one look at the thing, yelped, and ran away; my two juvenile arrowhawks each got off a single (successful) shot before they followed the bear.
But amongst our buffs were all the elemental protection we had, and that was good, because in two rounds the dragon did nearly 100 points of fire damage to most of us (none of us made our reflex saves).
Turned out, however, that the stone guardian is a golem, not an elemental, and it pounded on the dragon like a bad piece of meat. Because I wasn't prepared for the battle (having mostly fire spells prepared due to a disastrous misinterpretation of a divination earlier -- I thought the gods were ADVISING us to use fire, not WARNING us about fire), I spent most of my time galloping behind the back lines healing people, keeping them alive.
And astonishingly, between the sorcerer's barrage of magic missiles, the righteous-mighting-cleric's attacks, his cohort's hammer, and (most importantly) the stone golem's smackdown, we killed the red dragon. More amazingly, we did it without suffering a single casualty of our own (yay healing magics in battle).
True, the dragon didn't act optimally. But that's understandable: people snuck in disguise into her lair, cast a humiliating spell on her (Wall of Thorns, trapping her in place), shot at her a couple of times, yelped, and ran away. She was both royally pissed and convinced, from our behavior, that we would be pushovers: she therefore pursued us immediately, instead of taking the time to prepare herself for the pursuit. Had we been bluffing when we'd run away, she might have seen through our bluff -- but we were dead serious, and she realized it.
The battle was great fun. We were certain that at least one person would die, if not the entire party; indeed, had the cohort not rolled two critical threats (both unconfirmed) in his last round, the dragon would have killed the cleric that she was grappling, no question. As it was, we feel like we pulled ourselves out of the jaws of death.
Literally.
Thanks for the great advice, everyone! Even if I didn't get to use it this time, I've definitely filed it away for the next dragon fight.
Daniel