The Prismrivers are horrible to fight and Merlin is the worst of them in the bits where they aren't together. When I played PCB I pretty much always strove to deal the most damage to Lyrica so she would be the one I got to fight in the solo segments instead. She's so much easier.

And I agree, Sakuya A in PCB is the nicest bulletspread of any playable character ever. Which is awesome since she's also the coolest playable character by a fair margin so it's nice to be able to choose her while still getting an effective shot.