Mordenkainen's Swords work wonders against dragons. Cast a couple and send them close, but not next to each other, to protect at least one from the dreaded Wing Buffet. Since the swords are immune to physical damage, only breath weapons can do anything. Since they are real close, the stupid dragons will try unsuccessfully to beat a floating sword to death, leaving you to magic missile it to death, if you're feeling adventurous.
Silly Dragons.
