I went as far as lvl 43 (starting from lvl 12 - the end of my kiting-get moro run) with 1 black dragon and then I made them 6 because I managed to beat up the dragons in the dragon lair in montero. The black dragon is great against heroes and melee units because he's way too fast and cannot be "geysered" or shocked. This is why I have only lvl 1 Guardian Angel medal - you can't stone skin the black dragons
EDIT: BTW, the lone dragon can do it for quite a while without ANY backup. Backup is worth spawning if you're certain that your spells can't finish the remaining enemies off.
The same can be achieved with a call collosus spell and 1 red dragon or 1 troll and no kiting. I guess 1 bone dragon can cut it as well.