From the Super Mario Wiki, the Mario encyclopedia
Clandestine Cave is the secret tenth level of the Lava Pit in Mario + Rabbids Kingdom Battle. It can only be accessed by completing Bowser's Castle, then returning to Uphill Lava Trail to enter the cave and use Beep-0's Dig ability to remove some dirt blocking the way, then use the Push ability to push three blocks into place.
The first part of the level is an arena with three elevation levels and consisting of the Bwaaah Trio; Bwaluigi is the closest with a Supporter, Bwario is a bit further away, and the Lava Queen is the furthest with another Supporter.
The next part of the level features a rematch against Mecha Jr., as well as every single other midboss in the game. It is an area with cover checkered around it.