How would a leveled up expert from that same thread do?
One of the Experts would just convert the Dragon to his side before the fight even started via Lucid Dreaming.
The less silly Expert would at least be flying around doing poison strafe runs... with the Dragon's high Fort Save, that would be tough to land, but the Expert at least can be a threat and if he gets lucky with a poison shot he'd do some serious damage. The Dragon has a +17 fort save, and by level 12 the Expert can afford a +1 Splitting Longbow of Assassination (using Lesser Bracers of Armor for proficiency, they're cheap) to raise save DCs, plus he'd likely be using Sinmaker's Surprise at this point (since he could have Master of Poisons by then so the acid damage isn't a problem, and he'd like the better save DCs than Black Lotus). If someone cast Greater Magic Weapon on him the save DCs for his poison would be 29, otherwise it's 25. He'd have four attacks, but have trouble hitting (he's just an Expert, after all), though a 28 isn't impossible to hit. But if he landed a dose of his poison it would deal 2d6 con damage, which averages at 7, thus reducing the dragon's further saves and meaning he'd have a chance. Also, since the dragon has 21 HD, a single poison hit would effectively do 63 hp of damage in addition to the 3d6 acid damage. If he got lucky he could take the dragon down really fast this way... if his first hit lands poison, all the rest become more likely, and every successive hit increases the chances even further. Three poison hits would kill the dragon outright, so he's got a decent shot. Heck, it's theoretically possible that he just kills the dragon in the first round, though that would require hitting three times and the dragon failing a save three times (technically with amazing rolls two hits could kill the 21 con dragon, but that seems even more far fetched). Note that the dragon could take down the Expert very quickly... his saving grace is that he's attacking from a distance so he can avoid full attacks while delivering his own, and his mount is a lot more maneuverable (most likely, as I expect he's got a better mount by level 12. Desmoderu Warbat at the very least).
Note that I think a well made Monk could be useful too... but not the stupid skillmonkey ones that Giacomo made. You'd need someone that could be a threat to fliers and had the mobility to actually deal with the dragon (or the ranged attacks to do it).
JaronK