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