Handlock

Tips:

• This deck is pretty straight-forward. In my experience, cards like Mountain Giant, Twilight Drake, Voidwalker, and Acidic Swamp Ooze are very decent to keep in your starting hand. If you have both Ancient Watcher and Sunfury Protector in your hand, this can be a decent opener. 
• Know that choosing to mulligan cards can possibly give you Aranasi Broodmother, which loses some of its value, but it is still a decent taunt minion later in the game if you're behind. 
• Your best bet at healing is through Zilliax, Aranasi Broodmother, Rotten Applebaum, and Lord Jaraxxus. Tip: Use Plot Twist and/or Sense Demons while you have a Aranasi Broodmother in your deck to try to draw it and heal for 4 health.
•  If you are falling behind, cards like Twisting Nether, Shadowflame, Dread Infernal, Hellfire, and Mortal Coil can help you clear minions. 
• Sense Demons is a great way to draw cards to gain value from Mountain Giant and Twilight Drake.
• Don't underestimate the Ancient Watcher in your deck, it can be great with Shadowflame or Sunfury Protector. 
• Omega Agent is a good minion to play late-game to push for more board control. 
• Lord Jaraxxus is best played when you are below 15 health, as a healing mechanic.
• Remember, if you don't have enough mana or don't have the right cards in-hand to fend off an aggressive opponent, a gentle 8/8 Mountain Giant is a good choice to threaten them to back off for a second. 
• Lastly, don't be afraid to use Life Tap, because this deck greatly benefits from having a decent amount of cards in your hand.