I think what's needed is some de-leveled-listing, increase in HP, more varieties in their shouts, and smarter strategies (eg strafing attacks instead of landing on the ground). I think their damage is fine - they can one-shot my mage if he doesn't start healing himself right away.
EDIT: Making them immune to stunning is a bad idea. Stunlocking - yes, but not stunning. That just invalidates player choice. Perhaps it'd be a good idea to make it so that everything but the weakest dragons is only stunned for a fraction of the normal amount of time, ranging from 7/8 to 1/4.
And a giant should not be able to beat a dragon to death and then roflstomp the player. :/
Lore-based: Giants are supposed to be at roughly the same power level as dragons.