Which units would you like to replace/add?

Take another look at how much damage you dealt, and you'll realize that it's far from level 6! I recently fought phoenixes: they have 150 health, deal 30-50 damage, resurrect, and have a fire shield – their abilities level the playing field. The fact is that you've made the abilities too strong: double damage will sometimes be as high as 70-90, and with curse and duel, it will be stronger than all the others combined!
I wouldn't change the dragons; I would just tweak them a bit and give the upgraded version the ability to become ethereal.