To transform into Overlimit Chopper, two tasks must be performed: defeat Overlimit Chopper in Story Mode, and create the Rumble Ball. You can then transform during a boss fight by using twoRumble Balls (wait for one to run out, heal SP gauge, take another one). While you are using the second Rumble Ball, the enemy has to kill Chopper. Instead of dying, a cinema scene occurs. The scene shows Chopper taking a third Rumble Ball and he transforms into his Overlimit form, gaining a new HP and SP bar. Note: He isn't able to use items.