This is stage 3-11, the second to last level in the Mt. Snow region.
After the White Team wins the festival in level 3-10, "The Day Of," Cat Clancer appears and tells them that the Professor has been captured by Lunar and challenges her to a game of Dodgeball. Marina Liteyears makes the deal that if she wins, it will have to reveal the location of the Professor. The cat agrees that, if Marina can win in a dodgeball fight, it will take her to Lunar.
Defeat Cat Clancer at Dodgeball. Each competitor must stay at their own side with Marina being on the left side and Cat Clancer on the right side, as crossing the line is considered cheating.
Beat Cat Clancer without crossing the middle line and without taking damage, and the cat will reward you with the Gold Gem after talking to it.