Here is the full list of all 47 Danganronpa 2: Goodbye Despair achievements. It takes around 60-80 hours to unlock all of the achievements on Windows.
Unlocked all gallery items
Earned every other achievement in the game
Cleared a class trial without taking any damage
Saw every possible Island Mode ending
Cleared Island Mode once
Collected every possible present
Filled in every page of Chiaki's Report Card
Filled in every page of every character's Report Card
Filled in every page of Mikan's Report Card
Filled in every page of Akane's Report Card
Filled in every page of Sonia's Report Card
Filled in every page of Peko's Report Card
Filled in every page of Mahiru's Report Card
Hajime Hinata breaks through level 99
Filled in every page of Ibuki's Report Card
Filled in every page of Hiyoko's Report Card
Filled in every page of Teruteru's Report Card
Fully raised every type of pet
Filled in every page of Nekomaru's Report Card
Filled in every page of Nagito's Report Card
Filled in every page of Byakuya's Report Card
Obtained all types of items in vending machine
Collected 999 Monocoins
Collected every possible item in Magical Miracle Girl Monomi
Cleared all stages of Magical Miracle Girl Monomi
Finished every chapter
Found every Hidden Monokuma
Finished Chapter 5
Destroyed 500 white noise lines across all class trials
Filled in every page of Fuyuhiko's Report Card
Filled in every page of Kazuichi's Report Card
Finished Chapter 4
Finished Chapter 3
Destroyed 100 white noise lines across all class trials
Found half of the Hidden Monokumas
Hajime Hinata breaks through level 50
Break through 10,000 total number of steps
Finished Chapter 2
Filled in every page of Gundham's Report Card
Break through 5000 total number of steps
Finished Chapter 1
Cleared a class trial without having to retry once
Cleared a class trial without using your Concentration skill once
Challenged vending machine 100 times
Collected 50 unique presents
Fully raised one type of pet
Finished the prologue