Finally Together Achievement
Complete the game
You will need to complete every chapter in the First Book in order to complete the game. That means you need to beat at least six levels in each chapter. In chapter 4, once you beat your sixth level, the last level will unlock. Simply beat that level and you will have completed the game. Some of the later chapters get rather difficult, so take your time. If you are ever stuck, there are plenty of walkthroughs available on Youtube that will show you exactly how to complete each level.