The boss fought at the end of the PS3-exclusive dungeon, Lament. Svetovit can be pretty challenging - most of its attacks hit very hard, and it has an annoying tendency to play keep-away, making it tough to build Echoes. Bringing Beat along will let you more consistently build Echoes, and Salsa equipped with a Darkness Brooch will give you guaranteed access to Shadow Silhouette on the mostly-light battlefield. Polka is a good choice for healing (and/or Frederic, if he's learned Spirit's Pathway), Crescendo can tank Svetovit's attacks, and Serenade can be used for both healing and absolutely decimating the boss with a full Echo Meter.
This supplemental video is part of an ongoing Let's Play that you can read here:
https://talking-time.net/index.php?threads/crossing-the-bridge-between-dreams-and-reality-lets-play-eternal-sonata.8/