Here is the full list of all 12 GundeadliGne achievements.
Finish the game in Duet Play with both players alive.
Keep the holiday spirit alive.
Defeat every opponent in Demonic Challenge mode.
Defeat the true last boss.
Clear 25 lines in Eternal Babel mode.
Finish the game on Demonic difficulty.
Get 6 Alter Breaks in one game.
Finish the game with every character.
Finish the game without continuing.
Finish the game on Advanced difficulty.
Finish the game on Novice difficulty.
Finish the game on Standard difficulty.