Here is the full list of all 16 Cthulhu Saves Christmas achievements.
Fill all 4 item slots with an item.
Complete the game.
Defeat the 7th League of Christmas Evil member.
See a familiar face.
Forge an unbreakable R'lyehtionship.
Defeat the 6th League of Christmas Evil member.
Use a unite ability at 100% power.
Defeat the 5th League of Christmas Evil member.
Use the battle command.
Defeat the 4th League of Christmas Evil member.
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.