Some good advice in here already. I agree that this attack will most likely decimate your party if the dragon is well-played (as it should be).
Still, there are steps you can take to increase your chances of victory. First, find out anything you can about the dragon. Where is lives, what its habits are, who its
allies are. Scrying is a good option here, although the dragon will notice the sensor and know he is being scryed, giving him a warning. Consider whether or not you want to do this.
The only way to sneak up to a dragon (and even this isn't a guarantee of success) is to go in ethereally. Anything else is pointless, including invisibility, mundane hiding, silence spells, flying, etc. I recommend you scount the area ethereally via a gate to the ethereal plane. Be sure to prepare an escape in case the dragon spots you anyway (it is, after all, a high level arcane/divine caster). Try to see the dragon fighting, find out what spells he uses.
Once you've gathered information, get your party together and consider if you really are up to this. My feeling with the information you have provided is that you are not upto the task.
If the dragon does have allies, try to take them out before assaulting the dragon. If the lair is riddled with traps, use Etherealness/Teleport/Gate to get in or Find the Path.
Now, prep spells. Try to make the fighters mobile enough to keep up with the dragon. Air Walk or Fly help some. Mass Haste for everyone. The other spells mentioned also help.
Dispel anything the dragon has previously cast.
Have the wizard prepare to counterspell. It is unlikely that his blast spells will get through the dragon with any significance, between SR and its godly saves. Counterspelling a heal spell by the dragon makes
all the difference.
The wizard can also keep casting his spells thanks to his partial action from Haste. So make sure he buffs everyone. He should also prep some Walls of Force to reduce the dragon's mobility. While large dragons have huge fly speeds, they have low maneuverability. A Wall of Force in the middle of the air reduces the dragon's mobility even though he sees it, since it has bad 'minimum forward speed' and 'between up and down' (check the flight rules in the DMG). At the very least, such spells limit the dragon's actions. If he wants to get rid of a wall, he needs to cast Disintegrate or fly around it, and that's another spell, flyby or full attack you're spared of. Wingover only works once per round.
Dimensional Anchor until it gets past SR.
Anyway, you get the general idea: the spellcasters should work towards limiting the dragon's great mobility. This way your mobile fighters can go in and hack at him.
Summon a solar. Or three. Your 18th level cleric can and should do this. Offer it at least an equal share of the dragon's (considerable) horde. With its high fly speed, bow of slaying and 20 levels of clerical casting, plus a host of special abilities, the solar will give you an edge.
Find a way to neutralize grappling tactics. If it were 3.5, I'd suggest Freedom of Movement for everyone. In 3.0 I'm not sure what to tell you. Wieldskill (escape artist) on everyone? Grease?

The wizard shouldn't have a problem with this, since he can teleport/dimension door out (Verbal component only). The rest of the party--beware.
Boost everyone's will saves. Mordenkainen's Disjunction can put a real dent on a high level character's power. There's also no real counter for this cruel spell.
Prepare an escape route. I'd also prepare a secluded place where you can rest/regroup. Make sure it is somehow not scry-able. You want at least 24 hours before the dragon comes looking for you. I suggest you travel the planes for some time if this happens.
Even if all goes exceedingly well, I expect most of your melee fighters to die. The cleric and solar should keep an eye on their general condition vs the dragon's damage output. Heal/Mass Heal as appropriate.
Good luck. You're going to need it.