Break the ice face and expose the Shrewd Possessor
Flash the Workshop door panel to enter. The Possessor dives into the ice, and Luigi boards the sled-mounted shooter. Aim bombs at the plates covering the creature’s face. Lead the moving target so the bomb reaches a plate as it passes your aiming line. Clear the central plates while the face is close, then work around the outer edge. Once every plate is gone, send one more bomb into its open mouth.
Do not fire continuously at plates you have already destroyed. Taking too long lets the protection regenerate, and overheating the sled ends the attempt. In the arena, watch the portals that telegraph the Possessor’s charges. Dodge until it becomes vulnerable, then stun and vacuum it with a charged Power Surge. It returns to the ice after the first two exposures. The next shooting rounds add protection, so repeat the same measured aiming pattern rather than changing to rapid blind fire. Capture the Possessor after the third arena phase and take the Dark Moon piece.