The Brawl antagonist who uses the cataclysmic attack called Off Waves is Tabuu.
Tabuu is the hidden mastermind of The Subspace Emissary, Brawl’s cinematic Adventure Mode. He resides in Subspace and manipulates the conflict through intermediaries, including Master Hand and Ganondorf. His role is revealed late in the story, after the fighters have spent much of the campaign battling the Subspace Army and its commanders.
Off Waves is Tabuu’s devastating signature attack. When he first uses it, the energy waves sweep across the battlefield and turn the fighters into trophies, scattering them throughout Subspace. The attack demonstrates that Tabuu is far beyond an ordinary playable fighter or conventional boss.
The final confrontation adds an important twist. Sonic interrupts Tabuu’s later attempt to use Off Waves, preventing the attack from wiping out the fighters again. The heroes then defeat Tabuu and restore the areas consumed by Subspace. Bowser, Master Hand, and Ganondorf are major antagonists in the story, but none of them uses Off Waves.