The answer for this setup is basically luck, but also some unlocks and strategy of course. The pillars holding this up are:

• Pekero, a Legendary Joker which can only be acquired at random after finding a 0.3% chance The Soul card while opening a Spectral or Tarot pack in the shop. There are several, but this one is required for the effect to make a negative copy of a consumable item you hold after exiting the shop.
• Ghost deck, required to obtain spectral cards in the shop. It is otherwise incredibly difficult to obtain spectrals as a consumable, and you need to have Pekero make negative copies of Cryptid specifically.
• Cryptid, a spectral card which creates two duplicates of a selected card in your hand. When in a blind, this means you are temporarily increasing the cards held in hand by two as well.
• Brainstorm, a rare joker only unlocked after discarding a Royal Flush. Brainstorm copies the ability of the leftmost joker slot, meaning if you have Pekero there before exiting the shop, you can create multiple negative copies of a consumable such as Cryptid. Blueprint is similar, but for the joker to the right of it instead.
• Baron, a rare joker which makes Kings give 1.5x xMult while held in hand. In this case, the ideal setup is a red seal steel king, so the xMult effect triggers several times, alongside Baron being duplicated by Brainstorm.
• Joker duplication, as you normally only get access to one at any given time. This can be achieved through the destructive spectral cards at an opportunistic time (like Ankh when you only have one joker), or getting lucky with Invisible joker, a rare joker unlocked by winning a run with only ever having four jokers at one time. After two completed blinds you can sell it for a random duplicate. You can also hold onto Showman, an uncommon joker which allows duplicates to show up in the shop. Ideally you'd duplicate both blueprint and Brainstorm, but I assume this run didn't get the chance. You'd also normally have Mime, to further retrigger held in hand effects.

After all that, you'd just keep beating blinds, accumulating Cryptids, and whenever you're ready, duplicate an unholy amount of red seal steel kings and have like 100 kings in a ridiculous hand. Play high card, and let the fun start!