Assassin

The second boss you will face during the game, and although it isn't really powerful or a threat by itself, it has the special property of being completely immune to physical attacks. This is the reason why the battle against it also doubles as the tutorial for Purging, so just attack whenever the line in the Harmograph at the bottom of the screen is over a red beat to accumulate Beat Ups, and once the Purge Ready! notification appears above the Heart, execute the Purge and then either use Saki's Song Magic, or continue attacking with Aoto and Tatsumi, as the added elemental damage to their attacks can easily finish it off. However, keep on mind that this monster will be the base model for other similar creatures you will encounter much later.