Monster Island

This achievement is very simple to get. After completing any level with 3 stars, you can see the solution for free. Just click in to start viewing the solution and this achievement will unlock!

Very straight forward. If you are anxious about this achievement simply start a level, pause it and select see solution. You are given 3 at the start of the game and I recommend you to save it for harder levels. The other method is when you complete a level with three stars you get to see the solution for free! Here is a video on how to do it: