Hearthpwn

General mulligan: Sludge Slurper, EVIL Totem, Spirit of the Frog, Serpentshrine Portal

Against control (i.e slow Druid and Warlock decks), try and curve out into an early evolve and play a midrange game from there. Alternatively, you can manage your resources to commit to a Thrall, Deathseer gameplan, as the death knight has the ability to carry games vs control. Keep Desert Hare with Evolve. Corridor Creeper is often a good keep vs slow decks if you manage to find Lackey generators or the Hare. Other good cards are Brann Bronzebeard and Boggspine Knuckles (strong mulligan keeps vs control). Keeping Doppelgangster with Evolve is a risky decision against Druid as they can answer it with Poison Seeds, so probably only keep this pair vs Warlock. I shouldn't even need to mention how strong Devolve is to answer cards such as Voidlord and Spreading Plague to push damage with your evolved minions.

Against aggro (i.e Pirate Warrior, Paladin and Rogue), naturally you are unfavoured so your gameplan should be to go for an early evolve to try and win the board. Mogu Fleshshaper into Mutate helps to swing the board in your favour, otherwise Serpentshrine Portal and Devolve are your best cards if you don't land that early evolve.

If you are facing a lot of aggression I would suggest cutting Thrall, Deathseer and one Doppelgangster for two copies of Maelstrom Portal.