Draconians are a terrible race to use against Sky Drakes. Their two strengths are their flying mobility, which is inferior to what Sky Drakes have, and their breath weapon, which Sky Drakes are immune to. If you can cast invisibility, an force of Air Ships can probably do the job, though - it seems to me to be your only chance without branching out to another race.

Otherwise, try to conquer some dwarves or beastmen. They have some truly rugged and dangerous units that will be able to survive (at least briefly) Sky Drake lightning breath, and deal out significant damage in their retaliation. I cannot really agree with the recommendation of heroes. Unless they are seriously leveled and equipped with some godly gear, the sky drakes will murder them, and heroes are hard to replace. Whatever you use, enchant it to the teeth with the best unit enchantments you can get. Every little bit counts.

I seem to recall that whittling down a node in multiple waves of attacks hurts the quality of the reward you get for eventually beating it - that only the forces remaining in the node at your last assault actually count for determining how tough the fight was. I would not swear to it, though, it's just what I recall.