It would kind of be like Prolouge Pit in Paper Mario: The Thousand Year Door. You play the game normally until you get Shrink, and then you stop. You can use everything available to you before getting the final Blackrock key (execpt enemy drops, I'll get to that in a bit), and no glitches that would let you get things you shouldn't be able to get yet (the ones that come to mind are tab glitching and the glitch that lets you skip Cruel King entirely). However, Glitches and exploits that let you get the Shrink ability faster are allowed (such as the Dynamite skip). The run would start upon creating the save file and end once you reach the final floor of The Pit. Enemy drops, no matter if its a item or a card, aren't allowed, as that would make it very rng. You can still get enemy drops, you just can't use them.