After lots of testing with this deck I think there are 74 maindeck cards that are locked in and 4 market cards you cannot change. With 2 flex slots.
The combo exists by reducing the cost of Recurring Nightmare to 0 then combining it with Scattershot to ping for 1 damage. This triggers infiltrate allowing you to repeat for infinite damage.
The deck abuses the power system by only using 25 cards. You want to get as little power as possible in your opening hand as you will draw a lot of power with this deck. The 3 merchants allow the combo to be pulled quite reliably.
The main deck flex slot I have is Haunting Scream as it allows you to reuse a merchant or grab a combo piece.
The market flex slot of choice is Knucklebones. There is only rule with this. It must be a premium Knucklebones.
I like the current setup for more fun. I think the correct setup is Haunting Scream in the market over Knucklebones and shadow merchant in the main over the scream. But who doesn't love Knucklebones.
Any questions let me know.
Edit- deck changed to show the more optimal list.