Here is the full list of all 53 Gurumin: A Monstrous Adventure achievements.
Complete Crazy Mode with S or better ranking
Complete Beginner Mode with S or better ranking
Complete Beginner Mode with all levels completed
Complete Crazy Mode with all levels completed
Complete Happy Mode with all levels completed
Complete Happy Mode with S or better ranking
Complete Hard Mode with all levels completed
Complete Hard Mode with S or better ranking
Complete one level with C ranking
Complete one level with A ranking
Complete one level with B ranking
You'll get a shot at this picture during your second playthrough
Complete Normal Mode with all levels completed
For this picture, start by changing with Puchi in the house during your second playthrough
If you want this picture, start by wearing the maid outfit
Complete Normal Mode with S or better ranking
Chucky's furniture
Rocko's furniture
Pierre's furniture
Poco's furniture
Getting an Afro for Poco is the first step towards this picture
Getting a diamond from Pamela is the first step towards getting this picture
Complete one level with S++ ranking
Joke Glasses are key to getting this picture
The Cyclo and Storm Parts must be used together for this attack
Get the Master Parts for the most powerful attack
Putting the Pharaoh Hat on one of the monsters is your first step towards this picture
Pierre loves Sombreros. That's your first thing you need to know to get this picture
For this picture, find Grandpa's hidden Platinum Medal
Get the Wind Parts for attacks effective around and above you
Get the Beta Parts necessary for Parin's favorite attack
Complete one level with S+++ ranking
Complete one level with A+ ranking
Complete one level with S ranking
Complete one level with S+ ranking
Get the Alpha Parts necessary for this attack
Get the Storm Parts, necessary damaging enemies around you