This is only applicable to a high-level dragon that gets "good" spellcasting advancement, but fun nonetheless.
According to the Draconomicon, dragons that have reached the Old age category are allowed to take Epic feats that they qualify for.
Red Dragon + Antimagic Field + Widen Spell + Arcane Thesis (Antimagic Field) +
Permanent Emanation epic feat.
Pesky wizards trying to one-shot you with nasty spells? Now you can turn on an antimagic field at the end of your turn. When your turn comes up again, turn it off as a free action, act, then bring the AMF back on-line as a free action at the end of your turn. Lather, rise, repeat, dominate the world.
Note: Widen spell is required due to the fact that creatures larger that Large size are basically screwed when they try to use AMF. Sculpt Spell doesn't help, because that would stop it from being centered on you, and merely be a stationary effect with a range of 5 feet.
Versatile Spellcaster could probably help bring this on-line a bit faster, too.
Edit:
Actually, I don't think Versatile Spellcaster will help, since it has to be a spell that you know that is one level higher (technically a 5th level spell with a +2 metamagic applied to it is still a 5th level spell; thus you can't use Versatile Spellcaster to cast it with two 6th level slots?)Edit 2: Google has
proved me wrong.
Practical Metamagic (also from Races of the Dragon) would help you pull this off, however.
So, Versatile Spellcaster, plus either Arcane Thesis (AMF) or Practical Metamagic (Widen Spell) would get this down to a 7th level spell slot, which you can cast with two 6th level spell slots. Thus CL 12th.
This makes it obtainable for all the Chromatic dragons, even the lowly Great White Wyrm.
Edit 3: Found one flaw. Spellcraft (25 ranks required for permanent emanation) isn't an automatic skill for all true dragons. It is only an additional class skill for Blue Dragons. Unless there is a way to get Spellcraft as a class skill, this is generally unobtainable.
Edit 4: Loredrake to the rescue... Loredrake would allow you to qualify for Permanent Emanation earlier anyway, so you might as well just go that route. It would save you one of the above feats, and then you just take Improved Toughness to make up for the lessened hit die size.
Edit 5: The Keeper of Forbidden Lore abyssal heritor feat also adds Spellcraft (and Know: Planes) as a class skill. Sort of an odd feat, thematically, for a Dragon, however.