Mausoleum I is a fairly easy encounter that you’ll unlock after defeating Ribby and Croaks in “Clip Joint Calamity.” From the riverboat, go over the bridge to your right and then through the trees to the south. There will be a fish fishing and a creepy looking building. Enter the building to start Mausoleum I.
This is actually not a boss fight but it will grant you a brand new move if you are able to complete it. When the encounter starts, a bunch of pink ghosts will start floating toward the urn in the center of the room. You cannot shoot these ghosts, so jump on and parry them instead. This will destroy the ghosts.
Keep doing this over and over again until the ghosts stop coming, never letting one of them touch the urn. If you are successful, the Legendary Chalice will offer you a gift. Exit Mausoleum I she’ll give you Super Art I, the Energy Beam.