Hearthpwn

This is the deck I used for this month's legend climb, going 23-15 overall, including 12-4 vs Paladins. The deck really destroys Aggro and N'Zoth secret paladin, although Libram Paladin is more of a 50-50 matchup.
It's one of the best decks in the game and it's no wonder that in the current Grandmasters almost every single player brings it as one of their 4 decks and it is always the exact same deck.
I made two changes relative to that deck though, cutting both copies of Cult Neophyte and replacing them with 1 Venomous Scorpid and 1 Animated Broomstick
Neophytes are great versus No Minion Mage, Token Druid and OTK Demon Hunter but on my d5-legend climb I was barely seeing any of those. It was just Paladin, Paladin and Paladin with the 2nd and 3rd most popular class I faced being Hunter and Warrior. All minion-based decks where Neophytes are no better than a Bloodfen Raptor
On the other hand the Scorpid is a nice source of value and the broom has great synery with so many cards in our deck such as the Efficient Octobot, the minions summoned by Jandice Barov, the Venomous Scorpid itself and with Lifesteal or Poisonous Golems greated by Kazakus, Golem Shaper

Key cards are the following:

Efficient Octobot: if you play this deck long enough you will quickly realize how different ther games where you draw your Octobot early and manage to proc the frenzy feel compared to those where you don't. Whether you discount your big value bombs like Jandice, Kazakus and the Kazakus golem, your late game OTK combo pieces like Tenwu & Alex or a bunch of cheap battlecry and combo minions for a miracle turn with Field Contact, the mana cheating it offers allows some powerful plays to come way too early (or to be possible at all such as the Alex-Tenwu combo). It is quite similar to drawing an early Incanter's Flow for Mage in that sense.

Field Contact: The deck's primary draw engine. It can often draw you 5 or even more cards and that as early as turn 5 sometimes with some Octobot discounts and by using Shadowstep

Shadowstep: This is one of the most high skill cap cards in the deck as you only have two copies of it, but easily 10-15 targets that make sense for it and you have to know when to use it and when to save it for later. In matchups where tempo and board control is everything (e.g. versus Paladins) Prize Plunderer is one of the best Shadowstep targets for example. In slower matchups (Priest, Warlock, Control Warrior) you need to decide if you want to save them for some big value bombs (like playing Jandice Barov or Kazakus, Golem Shaper 2 or 3 times), use it on Mankrik to get multiple 3/7s that hit face as you draw through your deck or save them for an Alex-Tenwu combo in the late game. In other games you might Shadowstep a Wand Thief just to get a bit more value and/or draw one more card with Field Contact. Sometimes when my hand sucks I play an early Field Contact and Shadowstep him back to my hand to have him with 1 mana for a later miracle turn. Finally the Kazakus golems can often be great Shadowstep targets, either the 5/5 that summon a copy (for even more stats) or more commonly those that deal damage to the enemy board

Some general tips and tricks about the deck:
Foxy Fraud is in there because she is usually free stats. Being able to play a 3/2 and an 1/2 on turn 2 is very important in the fight for the board for example vs a Hunter. Vanessa VanCleef is actually mostly in the deck because it is a 0-mana 2/3 after you play Foxy and less for the value she creates
Kazakus, Golem Shaper is often played on curve and thus the automatic choice is a 5-mana golem for an on-curve play, but there are occasions where other choices make sense. While a 10-mana golem is usually too expensive even in the late game, it might be a game-winning pick on turn 4 if you are facing a Celestial Druid for example. The 1-mana golem is more often useful, particularly when you need to play it on the same turn as Kazakus and you are looking for a single target removal (poisonous - deal 3) or a +1/+1 buff to your board on that exact turn. It is also very important that (unlike the 5-mana one) it costs 0 once shadowstepped, allowing the poisonous - deal 3 to clear two targets for 1 mana on the same turn or the lifesteal - deal 3 to heal you for 6 and stabilize while at the same time you are winning the board (can be the difference between win and loss vs Hunter for example)
Keeping discounted pieces in your hand for a long time and playing for the Alex-Tenwu OTK vs Priest runs the risk of getting wrecked by Mindreader Illucia. I've had more success vs Priest by playing for tempo, developing board after board and killing them by turn 8 or 9

Mulligan guide: Keep Efficient Octobot, Foxy Fraud, Mankrik, Guardian Augmerchant if you have the octobot, Swindle or any other combo card if you have Foxy (Swindle is sometimes a fine keep on its own on Coin), Kazakus, Golem Shaper if you have a t2 and t3 play already and can naturally curve into him, Prize Plunderer sometimes if you go 2nd versus aggro

Matchup (based on personal experience and feel): There are no unwinnable matchups (nothing worse than 40%) and that's one of the nicest things about this deck. As said early, Paladins are easy, except for Libram Paladin which is probably slightly favored for them. Hunter is a tough matchup, you will always win the board by t5 or t6, but you will often still die as they have a lot of reach. But if you can survive till t8 or so or find some healing you will generally win. Rush Warrior is a tough one too as the deck doesn't really have hard removal and Brain Freeze and Prize Plunderer can't do much versus huge, ultra-buffed minions, but Control Warrior is a good matchup for us. OTK Demon hunter can often kill us before we can, but against other Demon Hunter decks we do great. Token Druid is around 50-50 and probably the outcome depends more on their draw than on our gameplay. Against no Minion Mage and Control Priest I feel like we are slightly favored, but it is close to 50-50. Finally Shaman and Warlock are bad and you should beat them if you are lucky enough to meet any.