Hearthpwn

20 upvotes and I'll write a guide. Cheers!

19/09/2017: deck updated for the post-nerf Koft meta.

 The deck still does well against the overall field. Unfortunately not much you can do about a turn 5 Giant board though.

Keep  mana wyrm, arcanologist and mad scientist as a general guide. Keep lackey with a secret but don't really play the lackey unless you need the tempo, otherwise hold for maximum tempo. Keep medivh's valet vs warrior, you'll need it to survive pirates.

Budget replacement:  Boom; + Faceless Summoner

Duplicate guide: Kabal Crystal Runner if you have the board, Azure Drake for the strong body, if you need tempo go for the 2/3 Valet. Ethereal Arcanist and Dr. Boom are good as well, other cards are just trash duplicates.

Secret guide: Don't play Counterspell when they have the coin. Kabal Lackey + Secret is ONLY a powerful play when you have a turn 2 to follow up, otherwise hold it. Kabal Lackey + Duplicate (Effigy) is not a good play, but since tempo is king in this deck, don't hesitate to play the combo against fast decks. Play Counterspell after they finish their quest. They'll likely not fall for it but they might have to delay their powerful turn, or if they don't have another spell bad luck. Against Secret Paladin, play Mirror Entity before his turn 6 or you'll lose the tempo battle. Against Pirate Warrior, try to Duplicate the Medivh's Valet. Against Egg Druid, play Counterspell as soon as possible, because board buffs are his strongest plays. Against control, play Duplicate on powerful minions and Counterspell to protect that minion or a big board. But if you don't have any other plays other than secret, don't hesitate to play them.

Secret techs: Potion of Polymorph can be swapped out for Mirror Entity and vice versa

Effigy swap with Duplicate

Effigy swap with Ice Block

 Actual footage of me playing the deck for reference: https://hsreplay.net/replay/8Nuas3Mz4kjUPEarSvy6zL

https://hsreplay.net/replay/WSkY3hTmuN2BcTuwJdandN (I made a critical misplay in this match playing a second Arcanist into Sylvanas because I was playing so fast. Luckily I still won. Don't do it guys)

I added 2 Effigy in the newest version, it's working great for pushing tempo. I have excluded Dr. Boom and added in Firelands Portal because of Boom bots anti-synergy. Some lists play Loatheb, some play Mana Wyrms, some play Unstable Portal. Lots of room for you to tinker around.