Hearthpwn

At first i have to say i crafted my deck out of this deck http://www.hearthpwn.com/decks/606186-heroic-magic-mirror-pacifier#related:similar-decks-collection.
But i did some changes, especially to get rid of some minions and most of the spells.

Mulligan

Try to get some of the cards like [card]Argent Watchman[/card], [card]Ancient Watcher[/card], Zombie Chow, [card]Silithid Swarmer[/card] or [card]Eerie Statue[/card] to get early heals and spam his board with useless minions.

Deck Idea

The idea about this deck overall is to spam MagicMirrors board with useless minions, so he cant attack, play new minions or use his secrets.
Always think about Mirror Entity, it will allow you to give him 2 of your minions instead of just a 1/1. Eater of Secrets will help you to get rid of the other 2 secrets (Counterspell, [card]Duplicate[/card].
The only thing you have to know is that he can burst you down with [card]Archmage Antonidas[/card] + Rhonin. There are 2 ways to counter them: 1.) Mill them out of his hand or 2.) fill his board completly.
Explosive Sheep is very good for a complete boardclear, if his board is too big.

Changes

I put out Shadow Madness, Shadow Word: Death, Silence and Entomb, because i think these spells are way to slow(you have to fill the board very fast) and Shadow Word: Death/Entomb are just countercards for [card]Archmage Antonidas[/card]/Rhonin.
So i put in a few minions like Zombie Chow, Explosive Sheep or Eater of Secrets. Some kind of techcards i guess, healing, clearing and popping his secrets.

If you do everything right you get this board :)

Good Luck !

Proof/Howto-Video