Why is it Garm is so hard with mage?!

[Diletante]

At Heroic, my warrior can kill it buck naked; also my hunter.
At LEGENDARY, I killed it with whatever I droped on quest. Most my gear was purple rewards. Didn't even had a full set bonus.

On the other hand, on heroic difficult, my mage simply can't put a dent on it, with a fully legendary gear. He's even wielding a Forbidden Knowledge. Still, the damn thing is unkillable. HOW COME?!
Why is it so absurdly difficult with mages?!

[papa jim]

forbidden knowledge is generally used with arcanist which was seriously nerfed several updates in the past, now it is elemental lords regalia with the grand magister command set. other things could be your abilities, frost beam or dfa are the most often used. you don't give us a lot of information about your build.