Beat the game to unlock the Premium Box, Adventure Review mode, Battle Review mode, and the Movie Viewer.
Master Fighter mode:
Get S-rank in all 50 Battle Review missions.
Hidden Boss Fight:
Finish all side-quests and talk to the boss who appears in front of the movie theater.
Intentionally fail three times and the game will prompt you to switch to easy mode.
Thanks to Revolution Readers Chapino and Loki!