FOREWORDS

Some thoughts and insights:

• First of all, this deck is not really difficult to master... but you have to well know the metagame especially during the mulligan phase.
• This is an attrition/control deck with a Mecha'thun combo package as a main Win Condition.
• From Rank 15-5 it still hoover around 55-60% win rate, with good matchup against almost anything. There's no auto-loss matchup since the deck has lot of consistency and different solutions.
• From Rank 5 to Legend there are currently too many SN1P-SN4P Lock around and they're really an issue for this deck (too fast, too consistent).
• Games are usually long and slow. Most of the time you need 10+ turns to achieve victory. So, you need to like slow/grind type of deck.
• Like all Reno decks, it's expensive to craft since it has lots of important epics and legendaries that are hard or impossible to replace.

HOW TO PLAY?

Early Game

• Draw cards, play on curve.
• Try to minimize damage.
• Make efficient trades.
• Try to be efficient with your AoE.

Mid Game

• Play high value minions.
• As soon as you have Bloodbloom, play Hemet, Jungle Hunter,
• As soon as you have combo pieces, play Emperor Thaurissan.
• Try to stabilise your life point using Reno Jackson, Antique Healbot and Zilliax.

Late Game

• Play discounted Mecha'thun, Bloodbloom and then Cataclysm and...
• ... Achieve victory.

MULLIGAN GUIDE

1. Mulligan for early drops, heal and aoe.
2. If you have Bloodbloom always keep it since as you draw Hemet, Jungle Hunter you can thin your deck by a lot, while also increasing the chance of draw the most powerful cards in the deck (like Kazakus, Reno Jackson, Twisting Nether and the like).

ABOUT THE COMBO

The combo is really simple.

1. You must have at least Bloodbloom and Mecha'thun in hand, while Cataclysm is a plus but not mandatory.
2. You have to play Emperor Thaurissan so you can discount both combo pieces (so the total cost of playing both Bloodbloom and Mecha'thun in the same turn is 10).
3. At turn 10+ when your deckpile is empty you just have to play Mecha'thun followed by Bloodbloom and Cataclysm.

ABOUT CARDS

About Hemet...

Apart from Emperor Thaurissan who enables the combo by discounting cards, Hemet, Jungle Hunter is the OP card of the deck. If you have Bloodbloom in hand and play Hemet, Jungle Hunter on turn 6, you will usually win the game. It's also the keycard when you face other combo or control decks.

About The Soularium...

By taking a closer look at the list, you will certainly notice that I do not play The Soularium. A lot of players consider that it is an auto include in Mecha'thun combo Warlock. This is not my case.
In fact, it really depends on which type of Mecha'thun Warlock you are playing. For sure, The Soularium is really a great card for the cycling version of the deck. I'm more doubtful about the inclusion of it in the controlish decklist I'm playing.
The Soularium was in the early version of the deck but I quickly realised that most of the time, it was a dead card in my hand because I never really want to discard anything aside from when I'm really at the very end of the deck (with like 5 or less cards left). So I removed it for another Area of Effect card: Demonwrath, who do wonders as another early game board clear.

About Zephrys...

It may be tricky to properly understand how to "force" Zephrys the Great to give you the card that you really want (or at last a good equivalent one), but with some practice and with the help of FrostyFeet's guide you will learn how to get the maximum value possible from it.

About tech cards...

You've got a couple slots for tech cards. In the current decklist you'll find Blastcrystal Potion and Deathlord but both can be swapped in and out for whatever you may like most, based on what you're facing while laddering. Kezan Mystic, Dirty Rat, Mind Control Tech, Big Game Hunter, The Black Knight, Tinkmaster Overspark, Gluttonous Ooze or any of the other notorious tech cards are ok.

I usually always keep Deathlord since it's a cheap big-health taunt minion that can be dropped in the early game to slow down aggro while giving you also some turns to prepare an answer (since its drawback will trigger only when removed).

TIPS

1. As soon as you have Bloodbloom in hand you must play Hemet, Jungle Hunter on turn 6. Playing Hemet, Jungle Hunter on curve is most of the time a game win (unless you are in a very bad position). This is why it's acceptable to keep both of those cards during the mulligan if you have them in hand, especially if you have The Coin. 
2. Beware that you must have at least 5 lifepoint when playing the combo or 4 if Cataclysm is discounted.
3. Beware of the board state and pop effect. Playing the combo with Voidlord on board is not a good idea because of the 3 Voidwalker that will pop. Better to sacrifice Voidlord then play the combo.
4. Against aggro decks you can win just by attrition, clearing their board almost every turn, healing up and exhausting their resources until you drop Bloodreaver Gul'dan to seal the match.
5. Versus Big Priest you want to play Hemet, Jungle Hunter asap or at last search for Kazakus and his polymorph effect to counter an early Barnes or Shadow Essence.
6. Mill Rogue is one of the worst matchup: try to dump your hand the more you can and avoid to use Life Tap. He will not put you under much pressure so you may have time to search for the combo pieces and, if you succed in that, just wait until they burn out your deck and then finish them.
7. For Secret Mage you need to use all of your healing effects at the right moment, while keeping always an eye on your lifepoints and remember how high can be its burst damage directly by spells. It's also really important to remove Aluneth asap or they'll just steamroll you.
8. If you're facing SN1P-SN4P Lock, I can only hope you get lucky because there is really not much you can do against it (expecially if they too run Mecha'thun).