|Darigaaz the Igniter|
How would you GM a Dragon attacking your party?
Step 1: prebuffing. If the dragon's attacking the party either it knows about their exploits through minions/allies/contacts or has spent some time observing them directly (from very far away) and any dragon worth their scales will plan accordingly.
Step 1a: if you hear them trying to prebuff (and you should), do so at the same time, setting up a counterstrategy. dragons should have huge perception and spellcraft scores, they will know what you're doing and how to best counter it.
Step 2: wear them down. Hit and runs attacks with spells and breath weapons. flyby attack. get in, blast them, get out, encounter over. come back several minutes later. you'll be burning through healing spells/items and wearing down protection from energy spells they might have. sic some of your minions on them in between flybys.
Step 3: most importantly, attack from unexpected directions. icy pass as you're a white dragon? pop out of the ice wall behind them. Rocky terrain for a copper? wall crawl like the demented spiderman you are. Forest for a green dragon? strike then fade into the foliage. Swamp for a black dragon? pop your head out of the water just long enough to strike, most parties hate underwater fighting and the water's surface provides you with total cover. Red dragon? light something on fire, use pyrotechnics for a smokescreen, use your natural ability to see through smoke to attack them, circling like a shark unseen.
Step 4: NEVER stand and fight unless you're sure you can shred one of them without being seriously injured in return. If they're important enough for you to warrant step one, you're not going to just land in the middle of them and blast away while their attacks bounce off your scales.
Step 5: Use the items in your hoard. Dragons may be vain (rightfully so), but they're not stupid. Even white dragons are smarter than your average human by the juvenile stage. A dragon's worth (among other dragons) is determined mostly by A) how big and strong they are; and B) the size and value of your hoard. Let me reiterate that: You are a dragon and what's in your hoard is just as important as how strong you are, there is no shame in using powerful items in it to augment your own power. You know what happens to dragons that are too proud to use their items? They get killed by those who aren't and their hoards become the property of their slayers.
Step 6: If all else fails, RUN. Living to fight another day is more valuable to your reputation than dying. A hoard can be rebuilt, stolen back. Your minions will still be out there for you to command. You will heal, you will plan, you will hunt them down make them pay. For you have seen them, know their faces, and have all the time in the world.