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