MArcomage

First of all: it's too expensive for its effect.
And even if it was cheaper - like 5/5/5 stock - it would be too weak for a rare.

I have to agree. Compare it with an rare attacker. I take for this... the Ether dragon:
Its non conditional damage/cost-ratio is 1:1,2 unconditional damage. If we take for 3 cost-cards a ratio of 1:1, you need at least 45 damage to have the ratio (Which would require 90 damage in one round, what finishs you most time.). Also this is conditional damage, which could require at least a higher cost-damage ratio. Also, this dragon is easyly summoned due the keyword and has also the banish keyword.

I made a concept like this a while ago i remember (And it was told to be too expensive for the cst of 25/25/25, while having an 8 times stronger effect than your concept):

http://arcomage.netvor.sk/?location=Concepts_details&current_concept=811

Anyway, balancing these types of cards is quite difficult, since you need to suffer damage (what you don't want) to play this card. Maybe lowering the cost to 5/5/5 and making it uncommon would help.

We already have a card called Avenger.

Actually, I wanted to add some keywords, like Far Sight. But, I guess I'll just deleted that concept card. I've forgotten there is already a card named Avenger.
And your Avenger of Heaven looks good =)