Here is the full list of all 9 Alone: Recode achievements.
Defeat 3 or more of each type of slime enemy in a single save file
Acquire all weapons in a single save file
Acquire all armors in a single save file
Discover all secrets in a single save file
Defeat all of the boss encounters in a single save file
Defeat Nightmare without battling any non-boss enemy troops on a single save file
Unlock the bonus ending scene of Alone: Recode
Use the song machine to replay some of your boss battles
Finish all conversation topics with your party at camp