From the start of The Bone Zone and after finding the Yellow Sign, head south/down following the large ? icon on the map screen (doesn't display on the UI/HUD). Then defeat the boss at the location. It will absorb other enemies, so try to prevent them from getting near it. I played as Ambrojoe with Magic Wand, King Bible, Santa Water, and Garlic (shown below) while circling around the boss to keep the area around it clear. Plan to start looking for the boss 5-10 minutes into the stage after you have farmed XP near the start.
Reroll and Banish help to narrow down level-up choices early on to seed your loadout. Once the boss is defeated and the scrolls collected, you will have access to the Secret menu.