This video shows what happens if you beat the Pit of 100 Trials twice in Paper Mario: The Thousand Year Door for Nintendo Switch. Whacka will appear on floor 100 after you have defeated Bonetail and go through the Pit of 100 Trials a second time, and challenge you to a boss fight.

NOTE: In order to challenge this boss fight, you need to hit Whacka on Keelhaul Key until he stops appearing. Some 30 minutes later you'll receive an RDM mail notifying you the battle is available.

Paper Mario: The Thousand Year Door Switch Whacka Boss Fight