Mulligans
General Mulligans
Try to get the combo as fast as you can on your hand.
Aggro Mulligans
Keep cheap bord clears like Defile and Hellfire to protect yourself. Vulgar Homunculus is a good taunt to protect with.
Midrange Mulligans
Defile, Hellfire and Spirit bomb are good spells to protect you with. Spellbreaker can silence a good minion and Vulgar Homunculus and Sunfury Protecter can make tempo.
Combo Mulligans
Get the combo on your hand fast and use Skulking Geist and Spellbreaker to make it harder for the opponent.
Control Mulligans
Get the combo on your hand but also try to have board control.
Draw out your deck as fast as you can with warlocks hero power.
Use 2 Galvanizer when you have Mecha'thun on your hand to reduce its cost to 8. Next turn play Mecha'thun , Bloodbloom and Cataclysm to destroy your battlefield and hand.
If your deck is empty, you win.