Here is the full list of all 9 Reimus Awesome Holiday achievements.
Uncover the identity of the Boss of the Red Company and beat them in battle
Recruit all optional party members (Mecha-Mayumi excluded)
Beat the game without recruiting any optional party members (Normal, Hard Mode or Lunatic Mode only)
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Get the secret Bad Ending
Avoid Backattacks throughout the entire game (A Backattack occurs when your party is positioned left in the battlefield)
Beat the game on Hard Mode
Defeat the secret Superboss on Hard Mode or Lunatic Mode
Beat the game on Lunatic Mode