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