This battle with Whacka is a new addition to the game in the HD rerelease on the Switch. It can be unlocked by completing The Pit of 100 Trials and Chapter 6. You will receive an RDM email letting you know that a fabled creature has been sighted at the bottom of The Pit of 100 Trials. I immediately attempted the battle as soon as it became available here.
This can be a very challenging fight as Whacka can unleash incredible amounts of damage by using Whacka Bumps that fall off of him whenever he is hit. Sometimes he will shake the stage and cause items to fall on him and create a large number of bumps for his next attack. Definitely try to bring a few Life Shrooms in case he gets some good hits in!
I personally found that using Koops' Shell Shield move works really well at absorbing the massive amount of damage he can do with his bump attacks. I tried as well to stick to hammer attacks so that I wasn't creating unnecessary Whacka bumps.
Upon completing the battle you get a Golden Whacka Bump. This is a key item that cannot be used (or eaten). Another bragging rights item similar to Prince Mush's belt.