Missable story trophy finishing up the Musician Arc.
After you've completed the showdown against your former friends, you'll enter Metaverse-Es to take down Aria yourself. She'll have one form but she can be a bit of a hassle given she's a final boss. Keep your distance and try not to build too much on the timeline at once, just in case she decides to plan ahead with some powerful attacks so you can block in time. Compared to the main story's final boss, this should be a cake!