You start with a large sphere hovering around a circle of purple glowing octahedrons as well as some sargasso looming around. Avoid the Sargasso and destroy the octahedrons which will cause a trap door below. If you keep running en route then you’ll reach a cutscene with Lucia who will hand you a piece of the Arcana after that you'll get a cut scene of Plutonian.
Basically, you can just stay back far enough out of range (approx. 2 of the floor tiles long) of its ball and chain and you'll never get hit. After a while some agonofinis will show up to help it, and laser beams will also appear. The laser beams are pretty easy to dodge, so just concentrate on Plutonian and you should be all right. You might need to run up to him to get him to stop twirling his ball and chain that deflects your shots. After that, take the elevator up and have a cut scene with Matier. She will ask you to go help Lucia with Arius.