Etna mode
Highlight the "New Game" option at the title screen, then press Triangle, Square, Circle, Triangle, Square, Circle, X. Etna's voice will confirm correct code entry. 
Alternately, complete the game after reading Etna's diary in each chapter. 

Unlockables
Bonus classes
Complete the indicated task to unlock the corresponding class.
Angel: Get Female Cleric, Knight and Archer all at level 100 or higher.
Archer: Get level 3 or higher in Bow Weapon Mastery.
EDF Soilder: Get level 30 or higher Gun Weapon Mastery.
Galaxy Mage: Get a Prism Mage to level 50.
Galaxy Skull: Get a Prism Skull to level 50.
Knight: Get Female Warrior and Female Mage both at level 10 or higher.
Majin: Get a Male Warrior, Brawler, Ninja, Rogue and Scout all at level 200 or higher.
Ninja: Get a Male Fighter and Male Warrior both at level of 10 or higher.
Prism Mage: Get a Star Mage to level 35.
Prism Skull: Get a Star Skull to level 35.
Ronin: Get a Female Warrior and Female Fighter both to level of 20 or higher.
Scout: Get two Fighters or Warriors, Males or Females each to level 5 or higher.
Star Mage: Get a Fire, Ice and Wind Mage all to level 5.
Star Skull: Get a Fire, Ice and Wind Skull all to level 5. 

Alternate endings:
Complete the indicated task to unlock the corresponding ending sequence.
Good Ending: Complete the game without killing any allies.
Dark Assembly: Persuade by Force 100 times in the Dark Assembly.
Etna Heroine: Kill 100 or more allies before Episode 5's Ice Queen Battle then choose to kill Maderas.
Flonne Tradegy: Kill 50 or more allies before Episode 3 and the Lavish Hall battle. Choose to Kill Hoggmeiser.
Overthrow Earth: Clear human world.
Bad Ending: Defeat the Item God on the 100th floor of a legendary item.
Midboss Ending: Lose to Midboss in your first encounter.