First go into act 4 of the Jiggoseum. You should see a giant swimming pool, you need to have a working boat or floaters on your trolley, or swim over to the other end of the flooded Jiggoseum and you will see Grunty.

Talk to her and go do the challenge! Select any vehicle! Boat or not and get out the vehicle when the game starts and swim because it is easier to move the balls!

Although grunt pops up in front of you and tries to stop you getting a goal and also sometimes bursts the nearest ball.
If grunt pops up in in front of you give a whack or two! Put 4 or more balls in the goal before the time runs out and get a Jiggy (or TT Trophy).