Step 1.
Start game and make sure settings is as follows;

• Auto-Forward Time -> Max
  
• Text Speed -> Max
  
• Skip -> All Text
  
• After Choices -> Continue

Step 2.
Start the game in 18+ mode.

Step 3.
Press skip and let the game process the full story. It takes less than a few minutes and when you're done you'll have earned all the achievements.

https://psteamproxy.yuanyoumao.com/app/416130/Written_in_the_Sky/