When Emil and Marta both have the skill "Special" equipped, they will sometimes do a special unison attack called "Arse Nova". This happens randomly at times, however if certain conditions are met, it will always happen. The key is in the elemental grid at the bottom left corner of the screen. When the grid shows equal parts dark and light, press C when the unison attack gauge is over half full as either Emil or Marta to do Ase Nova. If the Field Element is Light, do one base Dark-element attack (for example, Demon Fang with the skill "Tenebre" equipped) then do the unison. If the Field Element is Dark, do a weak Light attack (for example, Savage Reaper with the skill "Lumin" equipped) then perform the unison. Arse Nova is relatively powerful, and usually gets anywhere from 15 to 30 hits in as a part of a combo.
Emil's Ain Soph Aur secret Arte
After the Unison Bar is full, press B to do a Mystic Arte using Emil after using an Arcane Arte. Note: Emil must be in Ratatosk mode. Then, press B again before his Mystic Arte finishes, which is after his face's picture appears, to execute the Ain Soph Aur secret Arte. This is a Light elemental attack.
Mania difficulty
Successfully complete the game.
Mystic Artes
To perform your characters' Mystic Artes (Hi-Ougi), you must first have reached level 30 or somewhere around that level. Then, equip the skill "Special". With the "Special" skill equipped, perform either a High Magic Arte or an Arcane Art and hold B. Your character should then perform their Mystic Arte (Hi-Ougi).
Prequel bonus
Have a cleared saved game file from the original Gamecube Tales Of Symphonia on your memory card to begin the game with a random set of items.
Twilight Palace dungeon
Complete the Twilight Palace quests from the Katz Guild. There will be one of those quests available from Chapter 5 through Chapter 8.
Unknown mode
Successfully complete the game at least once, then complete it again with the Mania difficulty selected before all battles. Unknown mode is a harder difficulty setting.
Ase Nova unison attack...