Hearthpwn

 
On a 5 win streak to rank 7! That's in the top 4% of ranked players this season. I'm sure the deck can climb even higher on ladder but I just don't have the time to do so.

In the early game, you hope to gain control of the board by using your tempo-tastic one and two Mana minions - think of characters like Knife Juggler and Shielded Minibot, boosted by Coghammer. This is quite an aggressive deck in many ways, but it's important that you gain control of the board from the very off as well, and get efficient trades in.

Hopefully you draw into the deck's signature card Mysterious Challenger by the time turn 6 or so comes around. If you've done so, you'll not only put a ridiculous number of Secrets into play, you'll also draw down through your deck in doing so - making it more likely you'll get the other cards you want or need into your hand.

Mid to late game you have many powerful options and this is where the deck truly shines even after you drop Mysterious Challenger on the board. Many people expect you to run out of steam but with options such as Sylvanas Windrunner, Dr. Boom, Tiron Fordring, and Ragnaros the Firelord you can expect to have strong finishers.

Through a combination of Secret uncertainty on your opponent's part, and strong board presence, you should find yourself waltzing your way to victory if you can get Mysterious Challenger out onto even a mildly favorable board. I do not run ALL the standard Paladin secrets because I have replaced several with powerful mid-late game minions that will do more for you than a secret - even if played automatically by Mysterious Challenger.

Mulligan guide
As mentioned above, getting some kind of control on the board from the start is really important. A quick note on Secret Keeper before looking at the Hero-specific Mulligans below though. If you receive this card, along with either Avenge or Noble Sacrifice, then keep the combo - otherwise dig for something else.

Druid: Mysterious Challenger (with Coin), Haunted Creeper, Knife Juggler, Muster for Battle, Secretkeeper, Shielded Minibot

Hunter: Haunted Creeper, Ironbeak Owl, Knife Juggler, Muster for Battle, Secretkeeper, Shielded Minibot

Mage: Coghammer, Haunted Creeper, Ironbeak Owl, Knife Juggler, Muster for Battle, Secretkeeper, Shielded Minibot

Paladin: Coghammer, Haunted Creeper, Ironbeak Owl, Knife Juggler, Muster for Battle, Secretkeeper, Shielded Minibot

Priest: Haunted Creeper, Ironbeak Owl, Knife Juggler, Muster for Battle, Secretkeeper, Shielded Minibot

Rogue: Haunted Creeper, Knife Juggler, Muster for Battle, Secretkeeper, Shielded Minibot

Shaman: Coghammer, Haunted Creeper, Knife Juggler, Muster for Battle, Secretkeeper, Shielded Minibot

Warlock: Haunted Creeper, Ironbeak Owl, Knife Juggler, Muster for Battle, Secretkeeper, Shielded Minibot

Warrior: Mysterious Challenger (with [card]Coin), Haunted Creeper, Knife Juggler, Muster for Battle, Secretkeeper, Shielded Minibot

Combos and synergies
Here are the most important card combos to keep in mind as you play out the Secret Paladin deck. If you make any changes to suit the metagame, make sure you consider the impact they will have on your various deck synergies.
- No prizes for guessing that Mysterious Challenger holds the key to the most powerful synergy in this deck. Get this character out on the board around Turn 6, and you'll find it tough to lose.
- Secretkeeper is another card that plays beautifully with the mass of Secrets in the deck. Note, however, that this minion isn't boosted up by the Secrets enabled by Mysterious Challenger.
- Knife Juggler can be combined with Muster for Battle to get some random ping damage scattered across the board.
- Use the secret Redemption for value minions such as Shielded Minibot, Piloted Shredder, or a turn 6 drop with Sludge Belcher (assuming you didn't draw [card]Mysterious Challenger).

- Loatheb is most effective if you have solid board control which is easy to establish with this deck.

Secret Paladin counters
Secret Paladin can be a tricky deck to beat but knowledge is power. Good decks to field if you encounter a lot of these Paladins include Control Warrior, Freeze Mage, Dragon Priest or a Hunter with Flare slotted in for something dispensable.
You should also play intelligently and make sure you trigger the Secret Paladin's Secret spells in the right order. You should attack first with something that will survive Noble Sacrifice, for example, which will then proc Secrets like Avenge or Redemption. If nothing happens here, you can play a weak minion to catch the likes of Repentance. Just make sure you tread carefully and minimize losses at every stage, and you should be able to manage the situation.