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