Go into the casino, stomp the purple square. Now stomp all 3.

Spray him with water. He'll throw out fruit, bubbles or enemies. If it's enemies just kill them. If it's bubbles, get more water from them. If it's fruit throw first a hot pepper at him (the red things). Then throw a fruit at him. You have to do the fruit combos 3 times in a row before he dies.

It should be pretty easy!!

Super Mario Sunshine Walkthrough