It's all in the setup. Have the party meet a gnome with a newfangled quick-loading catapult in a heavy-duty fireproof (or acidproof, or whatever) cart, and when they're out and about have the dragon attack. Then it's a running battle as the party rides on a runaway cart with the gnome at the wheel (as it were) with the dragon performing strafing attacks (for whatever reason you see fit). Then when the party runs out of ammo they can fling themselves at the dragon, assuming you treat the aerial assaults as uber-charges for lots of damage.