Kill dragon while hes in the air, he wont breath fire until he lands. You can safely "ignore" the rider.. killing the dragon first is the trick here!

He will land at 20% hp so you just throw everythign at him then and hope for the best.

Good luck

edit: oh jeez all clothie? well Good luck!

well same deal here, try to tank the rider for awhile with your priest? while dpsing the dragon down to 25-30%.. then finish the rider and hope for the best