Something to seriously consider...
Have 'the big fight' take place with the Dragon under the effects of an
Anti-Magic Field. Granted this will strip the dragon of many of it's abilities, it will also rob the players of
thier biggest asset, magic. Take away a Wizard/Sorcerer's spells and they're in serious trouble, and it's a pretty hefty blow to a higher level Fighter's power-level as well. On the other hand, that Dragon
still has an impressive AC, BAB, and an impressive array of hit points.
As I'd recently read in P-kitty's
Story Hour you
can still use magic while inside a
Field, you just have to be tricky about it. A
necklace of missles works great if the target is outside the
Field's AoE, the same would hold true of Archers shooting magical arrows.
Someone mentioned the liberal use of Undead, this would support the use of the
Field in that the PC's are (nearly) crippled, yet the Undead would function normaly. Also, if you were to heed the advice of
MaxKaladin you'd have an excellent excuse to throw in some Constructs that the Dragon, or it's minions, had constructed. Perhaps even for use in this very situation.
Granted, this isn't going to be quite as effective as it might be becuase of the limited AoE of the spell, only 10' radius. Perhaps the Wizard you'd mentioned should cast it, and ride the Dragon. This would let the Wyrm get away from the spell (say to breath) and duck back inside if things get ugly. And even if it's the center of effect (ie: read a scroll, very old) it'd still be immune to magic and able to grapple someone and haul them underwater to drown. A
Dispel Magic can be resisted but
Anti-Magic Field can't
Hatchling Dragon