Mission Accomplished! Achievement
Clear all the Challenge Acts.
There are 90 Challenge Acts in the game; 45 for Classic Sonic and 45 for Modern Sonic. Each Zone has total of 10 Challenges with five for Classic Sonic and five for Modern Sonic. While completing the Challenge Acts, you will unlock the Bonds of Friendship achievement.
Each time you complete an challenge act, hit the bell and hit the music note to get an artwork or a song.