Armaggedon actual deals astral damage and then does burning as a post effect.
It also does less damage to yourself than the enemy.
You can get Ghost Armor and Crown of Chaos to reduce the damage even further.
You can get Crystal ball to reduce it more as well. I don't believe Guardian Angel does it though, but it might!
You can do solo Paladin Class with Black Knights or any other unit and Mind Tree Resurrection to do solo victories with Armageddon. Since Mind Tree Resurrection will resurrect the entire stack after battle, you go in with just solo Black Knights... afterwards your entire stack is revived.
That said, just use Black Hole instead.
Or use Death Star with careful aiming.