Mulligans
General Mulligans
You always want to keep the quest, and mulligan hard for Bloodmage Thalnos, Loot Hoarder and Dead Ringer. The deck needs to cycle hard to win.
Aggro Mulligans
Against Aggro, you can also keep early board control tools like Potion of Madness and Spirit Lash (which becomes more powerful when paired with Bloodmage Thalnos or Explosive Sheep).