Hearthpwn

Strategy

Midrange/Elemental Shaman is able to react to different match-ups. It has the tools necessary to shut down aggressive decks while being able to dominate the board and put other Control Decks under constant pressure. 

Mulligans & Matchup Specific Strategies

You should look for a good curve to play with. Fire Fly and Igneous Elemental are particularly strong choices as they provide additional Elementals to ensure synergy in the later turns as well as Jade Claws and Flametongue Totem to ensure early Board Control.

Against Aggro, you keep Taunts(Tar Creeper), Healing(Jinyu Waterspeaker) and Board Clears(Lightning Storm).
Hex might be good as well depending on the specific matchup.

Against Control, you should try to curve out as strong as possible. Play around their board clears and hit face as much as reasonably possible to set up some sort of burst finish. Use Servant of Kalimos as a late game refill or if absolutely needed to keep up the pressure. 

Timing Your AoE

While you shouldn't be too stingy with your Hex to have a strong board going into turn 6/7(Fire Elemental and Blazecaller can deal with a lot of s**t. You're Lightning Storms are precious and should be used wisely.
Against Aggro this sometimes is hard but try to delay it as much as possible, weakening minions before you pull the trigger and hopefully develop a taunt in the same turn because you will be Overloaded for 2.
Against other matchups you must make them think you don't have it and let them overextend into you to get as much value as possible out of the first one at least.
Unless you hold both, then it is sometimes advisable to go with a sub par first one to make them overextend into the second one. 

Leave your comments and suggestions down below ;)

Hope this is helpful and I apologize in advance for any typos or mistakes =P