Sonic Roar!
From the Super Mario Wiki
Bowser Jr. Roaring to his opponents.
Sonic Roar is Bowser Jr.'s Super Ability, used in the Wii game, Mario Strikers Charged. However, the move can only be used when Bowser Jr. obtains his special item which, like everyone else's special item, is depicted as his own head. Once executed, Bowser Jr. will roar incredibly loudly, and send a sonic wave in front of him. This shrinks all the players that it hits, both his opponents team and his own team. Any shrunken player will move much slower, and will be unable to catch up to normal sized players. The effects wear off over time. Sonic Roar acts similarly to Bowser Jr.'s Mega Strike, which also involves a loud burst of sound.
