TuDogz' advice was good. Pretty much assume as a spellcaster than 90% of what you throw at him will be useless. Assume he'll make every saving throw and that half of your spells will be stopped outright by SR. Also remember that an Ancient Red casts as a 15th-level Sorcerer, except he can pick Cleric spells like Heal, so any obvious vulnerabilities will be covered by his spells. To top it off, this dragon will have INT, WIS, and CHA all in the low 20s, so the DM should play him SMART. You won't just be facing him, you'll be facing the preparations he's made, the allies he brings along, and so on.
The dragon's big drawbacks:
1> Horrendous Touch AC. In 3E this was crippling thanks to all those saveless touch spells. In 3.5E, most of those have saves and/or have weaker effects.
2> Big size. In confined areas you can use this to your advantage; in open areas it just means they have more use for Reach.
3> DEX of 10. No need to worry about Combat Reflexes or anything. Incidentally, this is the first thing I changed IMC; most of our dragons have DEX of 18 with lower STR/CON to compensate.
4> And the biggest one: no matter what he does, the dragon is only one creature, capable of making only one offensive action per turn. (Well, two in 3E using Haste, or if he uses Quicken anything). If you can endure that one attack, you're safe for a turn. He won't be Readying many actions because of this, and if you set yourself up to counterspell him you can greatly improve the group's odds.
The bigger question is, what plans are YOU making? If that dragon is flying, stoneskinned, hasted, displaced, improved invisible, with Protection from Good and a few elemental protection spells up, how exactly are you going to stop it from just strafing you to death with hit-n-run attacks? As an individual you might have True Seeing on, but what about the rest of the group?
One correction to TuDogz' advice:
You can't Maximize a Dispel. A caster level check isn't a random numerical effect; it's like saying you can't use Maximize to guarantee an attack roll of 20 on a touch attack. This has been clarified several times, although I don't have a link offhand.
Also, if the dragon is smart he'll have Mirror Image or Displacement or something like that that adds a miss chance. If you blow your one 9th-level Chain Dispel on the wrong target, you're hosed.