how to deal with a dragons high attack roll

[Drachasor]

What you do is get some fantastic halfings to use you as a mount!

Or, more seriously, magic that gives a miss chance is good, just remember he has blindsight so some things don't work blindsense so they know your location within 60', but they still have a miss chance. Summons can soak up some of his natural attacks and provide flanking. They can also potentially aid another to increase your AC.

Breath Weapon is easy. Energy Resistance. Cover can also help. Try to not fight the dragon in the open so it can't fly. Remember, the breath weapon only averages 65 damage, so that's manageable if 30 of it is negated. There's also Energy Immunity, but I don't recall what spell level that is (hmm, if it is 8th you are out of luck).

What sorts of ACs does your group have? How much money to spend on creating/buying new items?