Mulligans
General Mulligans
Any combo card or duplicating effect is good on early turn since we want to assemble our combo quite early (turn 6-7).
Apart from that, having some removal is helpful to keep us alive before we have enough damage in hand.
Expired Merchant + Soul Barrage is the best thing to have in this deck. Just make sure you don't play it into any silence or devolve effects. (To avoid any interaction like that make sure to kill your Merchant the turn you play it so they can't get rid of the cards)
Aggro Mulligans
Against agro it is wise to keep 1 piece of interaction and 1 whipe.
This deck can do quite well at stabilizing early turns with that amount of cheap removal.
Expired Merchant + Soul Barrage is ideal since it can clear an early board and get us closer to our necessary damage.
Control Mulligans
Against control you can keep a lot more combo cards since we don't really have to worry for pressure on the early turns.
Going for a lot of card draw early is the main goal since we want to be able to combo off before any big disruption hit the board.
Scourge Supplies is a card that we can afford to keep against a lot of control deck.
A version of discard warlock that only focuses on assembling an otk combo with extra copies of Fist of Jaraxxus and Soul Barrage and then playing Talented Arcanist into Cataclysm to do +2 damage on all spells discarded.