Mask of CThun quest - try a Freeze mage deck. Stall out, use fireball and pyro to get your opponent low on health, then use a 3 mage mage spell and mask of cthun to win the game.
The main difficulty here is that going into your turn in the late game the opponent will usually have minions on board (which dilutes the mask of cthun damage), so be patient and try to get your opponent to 2 health without dying.
Not easy! Took me 20 games.