In floor 12 you have to shoot down the moon from the Spectral Catch. On the top deck of the Spectral Catch you can find a box with cannonballs and a cannon pointed at the moon. Smash the box and take one of the cannonballs to the cannon to fire at the moon. After a short scene you will get the notification that you unlocked the achievement for floor 12 “Shot down the moon from the Spectral Catch”.
Luigi’s Mansion 3 – Floor 12 Achievement – Shot down the moon from the Spectral Catch Video Guide
Luigi's Mansion 3 - Floor 12 Achievement - Shot down the moon from the Spectral Catch
