Victory! Partially due to some outstanding luck on our circumstances, but hey, that's how really tough fights can go.
We went in while it was asleep, and a Dungeoneering check told us that the sound of the magma bubbling and the scent of sulfur would probably dull the dragon's senses a little, so we spread out around the outer edge of the room. No surprise round though, because going near the treasure pile tripped an Alarm spell and woke it up.
The size was Huge, so it was either Young Adult, Adult, or Mature Adult.
The bard tried to talk to it at first, and convince it we were there for benevolent purposes, but considering the bard was standing right next to its treasure after tripping the Alarm spell, it didn't fall for it.
Before the fight, the Mystic Theurge had used Stone Shape to move some of the walls to form little cubbies and shields around part of the edge, so that we'd have somewhere to hide from its breath weapon. The druid also did some Animate Water magic and was creative with the use of that, covering every member of the party in a skin-tight layer of water, hoping it would block some of the breath weapon.
The very first round, the Mystic Theurge scored a critical hit with a Lesser Orb of Cold on a lucky Natural 20. The Halfling Charger was riding an eagle, and spent the whole fight diving down and moving past it with Ride-By Attack, so that she could harass it without being in range of its Full Attack.
The Barbarian used his Fire Immunity ring and dove right into the lava with it, hacking at it. The ranger plinked away with arrows and the Bard shot off some spell from Frostburn, little darts that were half cold damage and half phyiscal.
It breathed at the party, and the DM ruled that the creative use of the Animate Water spell protected us from it a little bit, reducing the damage so that some of the fire boiled the water off instead of charring our flesh. Flash forward a couple rounds and the Theurge has just taken a Breath Attack to the face, and is sitting at 1 hitpoint. He does a Dimension Door to one of the little cubbies he made earlier with Stone Shape, and the dragon turns its attention to the Barbarian, who hadn't been having a ton of luck breaking its AC. One Full Attack later and the Barbarian is sitting at 6 hitpoints left. Things are looking grim, and we're battered, but so is the dragon. The Halfling lands on the edge and her mount shifts into a War Horse, and she gets its attention with her trident, trying to ride circles on it, and taking hits from it. A round later and she isn't looking so great, but she does one last-ditch effort to win, doing a Charge at it and having her War Horse leap out over the lava to impale the dragon with her trident. The dice gods were kind, because she hit, and used her Deadly Charge, dealing something like 96 damage, which killed it to beyond dead.
That's not even the best part though. In the loot was the Deck of Many Things, where I witnessed the most astonishing piece of luck I've ever seen. The Bard elected to take three draws from the Deck. I watched the DM shuffle the deck, 22 cards, before and after each draw, and he showed us afterward, and it wasn't a stacked deck, all the cards were there. The Bard drew, THREE times in a row, the Moon card, the one that grants 1d4 Wishes. She ended up with 10 Wishes (4, 4, and 2). The DM just kinda sat there dumbfounded. She used them to spread rumors of her fame throughout the land (1), cast a couple Permanency spells on herself (4), and get straight cash (5). It was, to say the least, astonishing luck.