Welcome to the board!
First off, you can not dispel the effects of a heal (thank goodness, because if you could, one dispel magic would finish off my party
). If the dragon heals itself the only way to remove it is by damaging the dragon. (so a magic missile would help if the dragon didn't have too much magical resistance.)
I don't know if I have been lucky, but the three dragons I didn't leave alive never cast anything like heal. Volourn had the same, so I believe the chances aren't that big.
------------------
The last arrow of Legolas kindled in the air as it flew,
and plunged burning into the heart of a great wolf-chieftain.
All the others fled. -J.R.R. Tolkien, The Lord of the Rings