MArcomage

It's too devastating for just 8G/8R/8B and an uncommon. The cost:attack is almost double, the cost is easily achieved and not a great loss AND you have a secondary massive resource reducing effect.
37 attack AND you make the opponent's highest resource equal to his lowest one. And usually that resource is Bricks, since they are rarely used => opponent is crippled from building walls.

Well, you can say it about any of these 3 cards. Cost/attack of Shapeshifter, Earth Eater and Enforcer is even higher, and their resource reduction is at par with this one.
Maybe I'll raise its cost to 10/10/10 though.
But... I think I have a better idea.

There's a difference here. Shapeshifter, Earth Eater and Enforcer target a specific enemy resources and therefore might not always work. Your card will ALWAYS work, regardless if enemy bricks are more or less than enemy recruits (the example drawn from Earth Eater here). The cards you inspired from are very situational, whereas your concept will always deal attack on the opponent, and if it's played after the initial few turns of the game then it will surely be devastating by causing both effects

drop the durable tag and it becomes a decent rare