Go to Kakaboom Island (when it is time) and follow Fluffy. She will lead you to a big Magmaquin. Fight it with the nearby turret. It will first throw rocks; shoot them down. It should repeat this a few times, then it will turn around. Aim at its back and repeatedly press Square to fire a strong beam. Repeat this until it is dead. You will then get a piece of the Chassis. There will be two more Magmaquin. Kill them using the previous strategy to get the other two Chassis pieces. Note: Only Shadow Stones can be used on it.