Hearthpwn

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The basic strategy here is to survive while making 100% use of your mana in order to avoid Baron Geddon's free hero power: Ignite Mana which will deal 10 damage to your hero if you have any unused mana.  The deck emphasizes having tons of card draw to help you out with using all of your mana every single turn.

Things to be aware of:

• This is hardly even a Druid deck and could easily be done with just about any other hero and the exact same deck.  Substitute out the Druid specific cards for similar cards  like Azure Drake, Voodoo Zealot, and Earthen Ring Farseer and you have a 100% neutral deck.  Pick your hero power and go win!
• Once Heroic Baron Geddon has 3 mana, he can start to play his any one of his five signature Living Bomb spells when he feels threatened.  Make clever use of Youthful Brewmaster and Ancient Brewmaster to send the minion with the Living Bomb debuff back to your hand when you see it fit.
• Youthful Brewmaster and Ancient Brewmaster are amazing for this fight because you get to spend the mana of the returned card all over again which makes them extremely valuable on this fight.  Card draw is good for adding more of your deck's total spendable mana to your hand more quickly, but these Brewmasters' Battlecry add to your deck's total overall spendable mana.
• Chromaggus's effect is even powerful than that of the Brewmasters as far as adding to your deck's total overall spendable mana, but watch how many cards you have in your hand when he's in play or you may end up burning some cards or not benefiting from his effect.
• Don't let your health dip too low or Baron may go face with his Lava Shock, Lava Burst, Hellfire, or Fire Elemental.
• A glaring drawback to this deck is that the only 1-drop is Clockwork Gnome so it is an absolute must to mulligan for him or you will take 10 damage to the face on your first turn.

Synergies:

There really aren't too many synergies to keep in mind with this deck which makes it beautifully simple to play.

• Unstable Ghoul's Deathrattle pairs up nicely with Acolyte of Pain for some easy card draw.
• I picked Clockwork Gnome for the 1-drop specifically because his Deathrattle will give you another 1 mana card to play later which will be amazing in a pinch.

Mulligan priority:

Since this fight is about using all of your mana every single turn, the most important aspect to mulligan for is mana cost.  Don't worry if you do not get a 2-drop since you can burn your hero power to satisfy your second turn.

1. Clockwork Gnome is THE top priority for mulligans.
   
2. Any 3-drop or two 2-drops (Unstable Ghoul is great for dealing with Baron's early Flame Imps).
3. Any 4-drop

Possible substitutions based on your situation:

• If you need another 1-drop, I'd suggest Zombie Chow since there is no way that Baron will benefit from the Deathrattle and he's one of the stronger 1-drops.
• Azure Drake if you want some card draw with better health and attack.
• Voodoo Zealot or Earthen Ring Farseer if you want some healing that has board presence.

Cards you may want to substitute:

• Grove Tender nice because you have the option to either draw or gain a mana crystal, but both effects benefit Baron Geddon as well which can make things worse for you.
• Similarly, Coldlight Oracle is great for card draw, but it is equally as great for your opponent.
• Jeeves can help in a pinch, but since you will be using a lot of card draw, this may only serve to help Baron Geddon draw.  However, Jeeves can deal some whopping damage in the late game if you manage to survive until Geddon is out of cards.  Either way, you should win at this point, but Jeeves can make or break a close game.