Tommy J generously allowed us to host his decks and guides using (mostly) Basic cards to take down One Night in Karazhan Heroic bosses. Be sure to check out his YouTube Channel.
Unfortunately I felt that this boss wasn’t killable on heroic with a free to play deck, so I added a few commons to help with clearing the board which made it a lot easier. Having said that, I love the boss ability. Finding cheap cards that could be used to take advantage of it was a lot of fun.
So this boss has 3 secrets, all of which are actually fairly easy to play around (Mirror Image, Duplicate, and Counterspell), and can be used to your advantage. For example, say you play a Tentacle of N'Zoth. If it’s a Mirror Image, there’s now 3 potential aoe damage on the board. If it’s a Duplicate and you ping the newly created Tentacle, the boss will then have multiple Tentacles put into his hand. If it ends up being a Counterspell (which is worst case scenario in this example), you’ve still just dealt 2 damage to everything on the board.
If you do end up using this deck, I would recommend saving Explosive Sheep for Flamewaker, as two sheep do 4 damage to the board. Just don’t play too passively – remember that with all 6 of these commons you have several ways to remove minions from the board, and every extra card you add on top of that will only increase your chances.
Gurubashi Berserker combined with Unstable Ghoul or Tentacle of N'Zoth are an amazing combo. If you can play a Tentacle into a Mirror Image, then a Berserker you’ll both clear the board for your Berserker and massively buff it. One thing to keep in mind is if your Berserkers die off quickly you may have trouble killing the boss without him entering fatigue (which at the rate he draws cards is probably very possible).
Mulligan
Ideally you want to start with two cards to help you control the board (Unstable Ghoul for example).
Cards to Add
Abomination, (more board clear), Zombie Chow (nice potential heal), Deathlord (get some stronger minions out early). If you’re feeling adventurous Baron Rivendare or Majordomo Executus could even work (assuming the mirror can turn into Rag). Cult Apothecary are also a good pickup if you find you need healing (thanks /u/gwasp)
Cards to Remove
Water Elemental is fairly average, as is Lord of the Arena. Stormwind Champion would probably be the next I would take out.
One final thing for the Magic Mirror; if you were looking for a normal mode guide I wouldn’t use this deck since the minions spawn on your side. Use as many things with positive effects as you can!
A note – substituting vicious fledgling for razorfen hunter (if you have it) can lead to a very fun win condition. (giving it windfury is key!)
You put cards in this list that require you to have beaten the boss