Ayatane (boss)
The first among the bosses fought in the Rinkernator, Ayatane has lower defense than some of the upcoming bosses and all of his attacks are single-target; but to compesate, he has an extremely high attack power and is the foe with the highest critical hit rate in the game. All of his attacks are skills he can also use as a playable character: Spark, which causes average physical damage; Breaking Strike, which causes two hits of average physical damage; Dual Slash, which causes large physical damage; Hazy Shadow, which allows him to randomly block incoming attacks; and Roaring Blade, which causes extremely large physical damage, and most likely will one-hit kill any character it hits. Thankfully, he can't use either Godspeed or Illusionary Break.
As indicated above, he can randomly block any attacks you send him, so you'll have to rely on Song Magic and skills that hit multiple times if you don't want to waste HP and turns trying to attack him. Likewise, make sure to have a high defense to at least withstand Roaring Blade, and stock up on revival and healing items for whenever anyone is downed by his powerful attacks.