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