Mulligans
General Mulligans
You always want a Tracker and WM is the best secret to play on 2. I would argue nothing else is worth keeping except in very specific situations
This decks is simple, play The Marsh Queen on 1, into any secret on 2. Use Stitched Tracker to either discover Halaxxi or Subject 9 (you can cut Houndmaster Shaw to make Halaxxi more consistent, but Subject 9 is also a good draw and it is unlikely that you do not draw one of either Shaw, Halaxxi, Subject 9, or you 2nd Stitched Tracker before you play the first Tracker in which case it is still 3/4 chance to get Halaxxi).
After discovering (or drawing) Halaxxi, play all the cards in your hand to get down to 4 in hand before playing Halaxxi to ensure you can complete the Quest (alternatively, cut Shaw and Flanking Strike for two Fireflys).
Meanwhile, basically play Subject 9 / Spell Hunter to get you through. Zuljin should win you some games by filling your board with Spellstone & Animal Companion, playing a load of secrets, drawing up to three cards (unless Flare gets cut, and beware of milling with Trackings), and possibly hitting your opponent with a couple Deadly Shots or Flanking Strike.
The single Master’s Call is meant to be another way to discover Halaxxi (or Subject 9 if you miss). As for the single Flare, the inclusion of this card is mostly because of the Zuljin value, but it will obviously be heavily meta dependent whether this would be optimal. DK Rexar would be an obvious sub for either, but Rexar feels too slow and hand filling for this deck. The Quest & Zuljin should be more than enough value.
For Extra value, Zuljin should play the quest again. In the cases where you have discovered Halaxxi (which should be more often than not given the three cards dedicated to it), you could save the 2nd Halaxxi and do the Quest twice after Zuljin replays it.
This deck has a lot of ways to win.