Here is the full list of all 16 Cthulhu Saves Christmas achievements.
Complete the game.
Defeat the 7th League of Christmas Evil member.
Use a unite ability at 100% power.
Forge an unbreakable R'lyehtionship.
Defeat the 6th League of Christmas Evil member.
Fill all 4 item slots with an item.
Defeat the 5th League of Christmas Evil member.
Defeat the 4th League of Christmas Evil member.
See a familiar face.
Defeat the 3rd League of Christmas Evil member.
Recruit the old man, Belsnickel.
Defeat the 2nd League of Christmas Evil member.
Recruit the witch, Baba Yaga-chan.
Defeat the 1st League of Christmas Evil member.
Recruit the Snow Maiden, Crystal Claus.
Use the battle command.