Hearthpwn

Mulligan

It shouldn't come as much of a surprise that Awaken the Makers is a card to keep. If you expect your opponent to be running a control deck, consider keeping Raza the Chained and Shadowreaper Anduin. In general, keeping low-cost cards is the way to go. Only keep Twilight's Call if you have enough (i.e. 2) Deathrattle minions to back it up.

The game

Versus aggro/zoo

This deck is quite capable at dealing with aggro decks, if you get the right cards. Use Shadow Visions to get a copy of a removal spell. If you're low on board clears, go for Mass Hysteria or Excavated Evil. Need single-target removal? Pick Penance or Shadow Word: Pain. When you play Kazakus, choose Lesser Potion or Greater Potion and aim for Felbloom, Heart of Fire, Icecap or Stonescale Oil. Mystic Wool or Ichor of Undeath can also be useful, depending on the situation. You should only play Shadowreaper Anduin if you feel confident that you won't need Lesser Heal anymore.

You will often win against aggro decks simply by keeping control of the board. Playing N'Zoth, the Corruptor can also net you a win, even it it only summons 4 or so Deathrattle minions. It is even possible to ping your opponent down using Voidform in some rare instances.

Versus mid-range

I can't really give much useful advice here. 

Versus control

This deck has some tricks up its sleeve for versing other control decks too. Use Entomb to snatch minions that will serve you well. You can fish for an extra Entomb using Shadow Visions, too. Soul Mirror may just look like a glorified Lightbomb, but playing it against an opposing N'Zoth, the Corruptor or Bloodreaver Gul'dan can really turn the tables. On the topic of N'Zoth, you should try to get as much value from the card as possible. Hold on to it until you're confident that your opponent is out of big removal, or until you absolutely have to play it. Try to play Amara, Guardian of Hope when you are on low health, and play Reno Jackson only after you have your maximum health at 40. Getting value from Shadowreaper Anduin's Battlecry is not that important. It is often more beneficial to start slapping your opponent in the face with Voidform every turn.

Against control, it's a battle of attrition. You will not win against very greedy decks, but if you play things right, you can still win most control match-ups.

Versus combo

Pray to Yogg-Saron, Hope's End. He might not be in this deck, but you really don't have any options when it comes to beating combo decks.

All jokes aside, you can actually do well against combo decks if you're a bit lucky. Switch gears to a more tempo-oriented play style. Play Shadowreaper Anduin ASAP and toss Voidforms at your opponent's face until it violently explodes.