I have a secret for you Thyr! Did you want the first green skin, or would you be happy with one of the color variations? Because some of the latter unlock by using any of the hidden skins. I tank, so completing 100 dungeons, 200 world quests, and 1000 BG kills with my hidden Truthguard skin has unlocked all three of the color variations for corrupted Ashbringer, even though I've never earned the first. I guess those cheevos are all cross-spec.