Throughout the game, you'll come across a trainer named Actress Meredith. She'll ask permission to have a battle, and then spin around when she loses. You'll need to beat her in Melemele Meadow, Royal Avenue, and Ula'ula Meadow. After you beat the Elite 4 and finish the Ultra Beast sidequest, she can be found in Poni Meadow. Beating her one last time will grant you the Swords Dance TM.