Onslaught is the final boss of All-Star Battlemania 2. He is incredibly large and you will always fight him on the Final Arena stage.
Onslaught has two forms. In his first form, Onslaught will dash across the screen at regular intervals, firing a laser and small energy balls form his hand. Upon reaching half health, Onslaught becomes gigantic and hovers behind the stage. In this form, he fires energy balls from one hand and summons giant lasers from the sky. There is an audio cue right before he fires a laser, so watch out!
Beating Onslaught will complete arcade mode and role the credits. If you beat Onslaught with Sans, you will be challenged by Papyrus on Snowdin. Beating him will add him to your roster.