Mulligans
General Mulligans
Keep the Hand of Gul'dan in your hand at all times.
Keep the Expired Mechant and Nightshade Matron only if Hand of Gul'dan is in your hand.
Imprisoned Scrap Imp is good to be coined on turn 1 or played on turn 2.
Blazing Battlemage and Voidwalker can help you secure the board against early aggression.
Dark Pharaoh Tekahn feels good to be tempo'ed on turn 5.
Aggro Mulligans
Against aggro, you may have to play your minions on tempo instead of maximizing the efficiency of the Imprisoned Scrap Imp.
Do not worry about discarding the wrong cards with Nightshade Matron since your priority is to secure the board rather than having more value.
You may also want to keep Soulfire to deal with the opponent's buffed one-drops which mostly comes from their own Beaming Sidekick.