how to deal with a dragons high attack roll

[Immabozo]

Immunity to fire, fire resistance, evasion and imp. evasion are all answers to breath weapons.

Answers to attacks, as has been mentioned, miss chance (potions of improved invis?), not letting it get a get full attacks, etc.

Another option, optimize AC! Do you have a druid and a wizard? If not, just pay for buffs. Druid has Enhancement to your existing natural armor bonus spells (barkskin is one, I think), in addition to enhancement bonus to natural armor. So a +5 barkskin and another nat armor +5 spell is +10. Upgrade items for a few more, an animated +1 heavy shield is another 4 AC (screw mithral, this one doesn't even touch that casters arm!) a druid can get high nat armor in wildshape, hell, a wildshape druid can consistently stun him most rounds, probably.

Then there's Combat Expertise, fighting defensively, PrC (if you can take a level before facing him) for Con or Wis to AC (can get con to AC at least twice).

My level 12 wildshaping druid got low 70s AC (granted, after extensive research so that I could prove my DM wrong in tanking his made-to-loose encounter, took one all-day buff, 2 12 hour buffs, 1 10 minute buff and the first 2 rounds of combat buffing, so scrying was key). My level 15 fighter had 52 AC, so straight tanking a dragon is possible.

Although I have to say, the best defense, is a good offense. Do you have an ubercharger to charge it for about 100 damage? 200-250 if he is optimized and a little lucky with good rolls? A wizard to do a save-or-suck debuff? a cleric that can do another save-or-suck debuff? Or maybe several casters that can do a save-or-die, on 3 or 4 saves, the odds of losing 1 are much higher.