I found archdemons to be extremely effective against red&black dragons, but apart from that they are not all that great. Against dragons: 80% fire resist, high attack&defense, Rahha's shield (+50% defense against dragon attacks - looking at the stats, I think it just lowers enemy dragons' attack by 50%), Dragon Slayer sword (only 1, because of Feanora)... rather unfair
So I had a stack of archdemons in reserve later in the game, as you oppose quite a lot of dragons... but generally (I played as a mage), I preferred emerald green dragons as my fast attacker by far. With time back at the end of the 2nd turn, thats up to ~80 extra mana, depending on positioning (could be less or even more).