With this deck I defeated Magtheridon Challenge on 1st try.
Dune Sculptors, Arcane Flakmage and secrets will keep good AOE damage on board.
Arcane Amplifiers will give you 2 extra damage on Hero Power and once you get both on board you will be able to one shot Magtheridons Cubes.
Mirror Entity is super good at end rounds when Magthe is summoning super fat taunt minions.
Some spells can be switched for more effective too:
Messenger Raven, Blizzard or Arcane Explosion
Change these to other spells and you might get even better setup.