Magic Mirror draws lots of cards :) That's cool, because he'll fatique (easily).
You need to make sure he'll copy Tentacle of N'Zoth, Explosive Sheep or Unstable Ghoul with his secrets Duplicate and Mirror Entity. Use Frostbolt or Arcane Missiles to check if he's got Counterspell as a secret.
It worked for me in a couple of tries (like 4-5 times). It's a hard boss, so take your time. You don't throw on minions in early play. Do that in turn 3-4, in the mean time you just get rid of his 1-1 copies with your hero power.
GOOD LUCK! let me know if it worked for you like it did for me :)