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