To select any floor, complete the building 100%.
Free Battle mode:
To unlock Free Battle mode in which you can battle and previously defeated Boss again, beat Floor 13.
To perform the following trick, you will need two deck recipes, one named recipe A and the other recipe B. Go to the deck/bag screen, and load recipe B. Add the card you want to duplicate to the deck. Take out the card(s) (apart from what you just added) from the deck so that the number of cards is 50. Register the new formed deck recipe to deck recipe A. You will be prompted to overwrite the existing recipe; select "Yes". Load deck recipe B. You will now find the card you just added increases by the number you added.