Hearthpwn

As it's still fairly hard to make out what your opponent is running, I base my mulligan solely on the class they play with what I feel is the spec I face most often.

General:
SI:7 Agent, Earthen Ring Farseer
With coin: Edwin VanCleef

Warrior (control):
Tomb Pillager, Azure Drake
Honorable mention: Eviscerate /Shadow Strike

Shaman (midrange/aggro):
Backstab, Sap, Eviscerate
Honorable mention: Bloodmage Thalnos if you already have Backstab

Rogue (N'zoth):
Backstab, Tomb Pillager
Honorable mention: Eviscerate

Paladin (N'zoth):
Backstab, Tomb Pillager
Honorable mention: Eviscerate , Sap

Hunter (midrange/N'zoth):
Backstab, Eviscerate
Honorable mention: Sap, Tomb Pillager

Druid (C'thun):
Backstab, Sap, Tomb Pillager
Honorable mention: Eviscerate ,Shadow Strike

Warlock (Zoo):
Backstab, Fan of Knives, Tomb Pillager
Honorable mention: Shadow Strike

Mage (Freeze/Tempo):
Backstab, Eviscerate, Tomb Pillager, Azure Drake
Honorable mention: Bloodmage Thalnos if you already have Backstab

Priest (C'thun):
Backstab, Tomb Pillager, Azure Drake
Honorable mention: Bloodmage Thalnos if you already have Backstab / Eviscerate

Cold Blood: amazing value, you can use it on any minion to remove a big threat or do a lot of damage to the face, great with Conceal.

Conceal: secure your board, great with Gadgetzan Auctioneer, Violet Teacher, Edwin VanCleef or a minion with Cold Blood cast on him.

Edwin VanCleef: strongest rogue card in standard imo as there are almost no silences being ran. Build it as big as you can, but don't waste valuable spells just to make him bigger.

Fan of Knives: MVP vs Zoolock, even better if combined with Bloodmage Thalnos.

Shadow Strike: To many big minions out there that Sap and Eviscerate alone don't seem to cut it. Might cut it out of the deck though.

Xaril, Poisoned Mind: The body might suck, but the toxins make up for that easily (+3 dmg, 1 mana Shadowstep (which is amazing with Leeroy Jenkins), a card draw or a 1 mana stealth for whatever minion might need it!)

Bloodmage Thalnos: Cheap spell damage and a card draw.

Violet Teacher: Fill that board with tokens, not that many AOE being ran now and the 5 HP body is good too.

Cult Apothecary: This card is great when you're behind vs Zoolock/Shaman and has saved my life plenty of times vs any class that had more than 3 minions on the board. I see a lot of other Miracle decks running Earthen Ring Farseer x2. It's definitely an option as it also gives you a turn 3 drop (which this deck lacks), but for me it came down to either cutting Cult Apothecary and 1x Violet Teacher/Shadow Strike. I tried it, but I felt the current version is better.

Questing Adventurer: he's never less than a 4/4, which is damn good for a 3 cost minion. If the opponent can't deal with him, he's going to have a hard time. Same rule goes here: build it as big as you can, but don't waste valuable spells just to make him bigger

Leeroy Jenkins: Into the toxin, into Conceal, into Cold Blood, he's amazing.

The other cards are self-explanatory, but if you still have a question, feel free to drop it in the comments.

Why do I not run that card:

Emperor Thaurissan: To slow for this deck

Shadowcaster: he could be good with certain cards from the deck, but I don't think he's worth it, definitely not with the amount of 5 mana drops in this deck atm.

Blade Flurry: Sucks right now, also no Deadly Poison in this deck.

Argent Horserider: Another card I really wanted to include, but didn't know what to cut for, maybe Shadow Strike.

Earthen Ring Farseer: Need to run 2x to have enough healing, decided to go with 1x Cult Apothecary.

Deadly Poison: definitely a solid include, but I never really felt like I needed it.

Journey Below: Nothing I feel I can cut for it.

Shadowstep: I used to run it 2x than 1x and now not anymore, it ends up being a dead card to often, in a deck that already has plenty of situational cards, it wasn't worth it for me.

Shadow Strike: He's been in the deck for quite some time, but after a lot of testing, I found I'm better off without it. But still a solid include in the deck if you want to.

According to my experience so far:
Tough match-ups: Aggro Shaman and any type of Warrior.
Easy match-ups: Zoolock 
Equal (or only slightly favored/unfavored) match-ups: anything else

In theory it's fairly easy: you fight for board control until you can do your miracle turn.
During your Miracle turn you either try to get lethal, or set up for lethal in the next few turns. 
Although it sounds easy, this can be fairly hard to achieve, as any wrong choice early game can very well end up costing you the game in a later stage.
E.g.: Do I prep int eviscerate that minion, or do I save it for my Miracle turn?

There are a few things to keep in mind with the use of spells to maintain board control early game:
- Can his minion(s) trade favorable with my minion(s)?
- Do(es) his minion(s) have an effect that really needs to be removed and I can only remove it/them with a spell?
- Is the face damage I'll take if I don't remove his minion(s) too crippling later on?
- Do I have anything on my board that improves the use of my spells (e.g.: spell damage)?
- Do I have anything on my board that benefits from me casting spells (e.g.: Violet Teacher).?
- Would he waste resources next turn on one of my weaker minions?
If you answered YES to any of these: it might be worth it to use a spell

- Can I solve this problem by using any of my minions instead of multiple spells (e.g.: SI:7)?
- Can I solve this problem using my dagger?
- Can he remove my stronger minion(s) easily with cards he has in his hand?
If you answered NO to any of these: it might be worth it to use a spell.

It'll still totally depend on the situation and the deck you're facing.


So you have survived the early game, what's next?
On a moment your opponent board isn't as strong anymore (because you played your early game well and didn't make mistakes). You'll throw down your Gadgetzan Auctioneer and start your Miracle Turn. 
If you can't get a lot of value out of him the turn you place him on the board, make sure you protect him one way or another. The best ways to do this is using Conceal, preferably with more than just the Auctioneer on your board. The stealth Toxin works as well. If you won't protect him he will be dead next turn and odds are you'll lose that game.
After your Miracle turn a lot of minions on your board will be must kills for your opponent (Cold Blood x2, Leeroy maybe, huge Edwin, Violet Teacher), you try to protect those minions with whatever you can while dealing huge damage to the face with your buffed up minions.

Congrats, if you made it until here chances are pretty big you've won!

Combos:
If there are spells involved in these combos most of the time you only want to do it if the spell(s) is/are actually useful.
- Gadgetzan Auctioneer (+ Preparation) + spells (duh).
- Bloodmage Thalnos + Backstab for those pesky 3 health minions.
- Bloodmage Thalnos/Azure Drake + Preparation  + Fan of Knives.
- Backstab + SI:7 Agent/Edwin VanCleef(often worth it to keep your backstab until you can play them together).
- Violet Teacher + Preparation + Spell (+ Conceal).
- Preparation + spell + Edwin VanCleef.
- Leeroy Jenkins + toxins (most of them are actually great on him, definitely returning him to your hand).
- Questing Adventurer + coin + conceal / (+ prep) + spell + conceal 

Small tips:
- Don't drop Bloodmage Thalnos if there is no use for him (no spells, you don't really need the draw).
- Coining into Tomb Pillager is a great turn 3.
- Don't waste spells.
- Don't be afraid to use spells. (see what I did there)
- Don't save Cold Blood only for Leeroy Jenkins (or other burst) (on a Violet Teacher token it's great to take out that pesky 5/5 minion.)