And here's another thing: immunity isn't that great – you can't even cast Blessing, and the larger the army, the less combat skills matter.

Are you just trying to be funny? If you're talking about an Archangel and a Black Dragon, why would you cast Blessing on the Archangel? It won't do anything :D, and regarding the army, do all your creatures in the army have full immunity? And that's definitely NOT bad!!!

P.S. Also, the Archangel has a chance to attack twice without taking damage (+2 morale vs +1 for the Black Dragon).
And the more creatures there are, the more Lightning Bolt is needed...

Be clearer, it's not clear what you meant when you said "the Archangel has a chance to attack twice without taking damage," a dragon can do the same, and why are you writing "+2 morale vs +1 for the Black Dragon"?