Hearthpwn

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MULLIGAN:

You usually want low drops and curve out, Mecharoo, Whirliglider are the top priority. You want to cast Tracking to look for certain cards at certain moments, like Leeroy Jenkins to reach lethal, Unleash the Hounds against token druid or just to help your curve.

Most of the time, I mull anything above 3 mana and Nine Lives (you want this later in the game). 

Against Warrior: you can keep 4 drops like Replicating Menace or Explodinator, even Boommaster Flark.

Against Rogue: I have to be honest, its a nightmare matchup for us. Spider Bomb is necessary to try to compete for board. 

Vs Token druid: mull for low drops and Unleash the Hounds. Around turn 6 and 7, you should have a decent board, it's time to go face because you can't compete with the Twin spell druid's have.

Vs "Big" mage: keep 1 Venomizer and/or 1 Spider Bomb to neutralize the turn 4 Mountain Giant, then SMOrc all the way.

TIPs
If you see you can't retain board anymore, hold Missile Launcher and a Venomizer to clear the board and make an epic comeback. So, have that in mind when you spend the coin but still play your minions to stall until that turn.