Theatrhythm Final Fantasy/Chaos Shrine

After playing the other two modes, you will receive a Dark Note, and will be able to play Chaos Shrine mode. This unique mode tends to grant less Rhythmia than other modes, and high scores are not recorded, but it makes for it by granting lots of experience points and key items.

Dark Notes
Dark Notes are random combinations of one FMS and one BMS. The exact songs are only revealed after beating them, so new notes are a challenge. Furthermore, aspects of a song, including difficulty, are randomized and constantly change throughout the song, so this mode in general is more erratic than Series and Challenge.

There are three bosses for every Dark Note. Your characters' stats and your performance will determine which boss you meet. After the initial unlocking of Chaos Shrine mode, Dark Notes can be unlocked by beating an existing note for the first time or by trading for one in StreetPass.

Bosses
Each Dark Note has three playable bosses. Which boss you fight depends on how well you do throughout the two songs. By default, you will face Boss #1, which has a one in three chance of dropping one of the crystal shards necessary for unlocking characters. However, by progressing far enough in the FMS, bosses #2 and #3 become available. In the BMS, after defeating the first two enemies, the game decides which boss you get to fight based off of how much HP you have remaining. If you have under 75% of HP remaining, you fight Boss #2, which has a two in three chance of dropping a crystal shard. Conversely, if you have 75% or more health remaining, you get to fight Boss #3, which can drop one of three shards. That said, the boss always needs to be defeated before the shards can be gathered.

In order to maximize one's chances of getting a crystal shard, have a character with high Agility lead your party in order to speed through the FMS. If you think you'll mess up, make the first two party members speedsters. Meanwhile, you'll want to have characters with high HP, Attack, and Magic for the BMS. If you can, try to defeat the first two enemies before you do the Summon Feature. That way, you can use its powerful attack to wipe out the enemy boss.