Mulligans
General Mulligans
It's always good to keep Duskbreaker just in case. If you know you're not facing aggro you can keep Spiteful summoner or keep it if you already have solid early drops in hand.
Aggro Mulligans
Duskbreaker is always #1 card to mull for against aggro, same with Netherspite and Northshire.
Basically spiteful priest, but before turn 6 you try to curve out your opponent, then finish them off with a 8 or 10 minion summon on turn 6.
Shadow Ascendant, Kabal Talon, Saronite chain, help you keep minions on board going into turn 5
Spellbreakers for silencing doomsayers, combo priests and voidlords