Deck CompositionThe core cards in Midrange Druid are as follows:
2x Wild Growth
2x Savage Roar
2x Keeper of the Grove
1 or 2x Force of Nature
1x Zombie Chow
1x Big Game Hunter
2x Shade of Naxxramas
2x Piloted Shredder
2x Sludge Belcher
1x Emperor Thaurissan
1x Sylvanas Windrunner
1x Dr. Boom
2x Ancient of Lore
Mind Control Tech
Druid of the Claw
The Black Knight
Piloted Sky Golem
Ragnaros the Firelord
The deck centers around the ramp effects of Innervate & Wild Growth as well as the finishing power of the Force of Nature/ Savage Roar combo. The rest of the deck is comprised of difficult to deal with minions to apply pressure to the opponent and force them to deal with the board or lose to the combo. Now with the addition of Emperor Thaurissan the ability to combo out the opponent on earlier turns makes the deck even more powerful.
Playstyle, Tips, and NotesThe early game is reliant on mana ramping cards and can be slow if these are not in the opening hand. Often there will be the ability to play a large minion early but do not do so at the expense of the next few turns. It is better to play a bit slower if it allows you to drop something each turn so avoid using innervate on a big turn 1 play if you cannot play something on turn 2.
In the mid-game you will want to be dropping strong threats such as Piloted Shredder and Azure Drake to put pressure on the opponent. At this point if you do not have control of the board it can be difficult to gain it back so it is important to make trades that allow you to keep the initiative if possible. If you cannot remove the opponents board then it is usually better to develop your own rather than spend your entire turn clearing minions.
In the late game you will usually have some part of your combo and are looking to draw cards using Ancient of Lore if you do not. At this point setting up a potential lethal combo play is the goal. Cards like Dr. Boom are great for doing this as the extra bodies are what you need to add a few points of damage. If you have played Emperor Thaurissan at some point then your combo pieces are cheaper and can often lead to surprise burst damage the opponent can't play around. Get that card down as soon as possible.
One tricky component to playing the deck is how to use your Shade of Naxxramas. Keep them stealthed for a few turns but avoid getting too greedy with them as you can end up getting overrun. If a value trade presents itself or you have reached around 5 power it's probably best to reveal them and take the damage. This is not a perfect science and a call you will have to make based on the board and game state.
MulligansThe general rule is mulligan everything except Innervate & Wild Growth as these are the most important in the early game. If you have either of them then keep cards like Shade of Naxxramas & Piloted Shredder. Zombie Chow is also a keep against pretty much everyone as it is nearly worthless in the late game. The chow is in the deck to help with th face hunter match-up.
Combos and CountersThere are a few cards that play well with each other in Midrange Druid:
Innervate + any midrange creature
Savage Roar + Force of Nature
Azure Drake + Swipe
There aren't a ton of big combo plays except for the big one. This deck is more about dropping threat after threat and keeping constant pressure on the opponent but look for big innervate plays that will get you out front of the opponent and take a tempo lead.
Control Warrior: a fairly easy match-up. Since the Warrior has to spend so much health clearing your minions with weapons you can usually combo them out fairly quickly in the end game. Be careful over-extending into Brawl but you should win around 70% of these games.
Hunter: Face Hunter can be scary but if you can outrace them if you can get a Savage Roar. Zombie Chow on turn one can shut them down fairly well and if you find that you are losing to them a lot you can tech in a Kezan Mystic.
Paladin: This can be tricky since they can often match you threat for threat. A big innervate play can fall to Equality or Aldor Peacekeeper and can end up losing you the game. If you can stay ahead on board and hold your Keeper of the Grove for their Tirion Fordring then you should be okay. Teching in Harrison Jones and The Black Knight will help with the match-up
Rogue: Against Oil Rogue you can pressure hard and burst them down as long as you don't get destroyed with a big Blade Flurry. Remove their high threat targets and play aggressive when you can set up lethal. Against the new Mill Rogue you should be able to run them over by just playing out your hand in an aggressive manner.
Warlock: Demon Lock has some big threats of their own with Mal'Ganis (which you save BGH for) but their early game is weak and if you can silence the Voidcaller then you have taken away one of their biggest tempo plays. Handlock will get low on purpose so I actually just don't attack them once they are under 20 and set up for one big burst turn. It's a fairly easy match-up when played that way.
Priest: This is an easy match-up and one that you should win about 80% of the time. Your threats are all 4-attack creatures and they play such a reactive game that you should be able to draw into your burst and just win from there.