The Sun and Moon

System Nine consists of levels 121 - 140 plus boss level IX. Remember that there is no time limit for completing levels for this achievement, so take your time so you don't needlessly die over and over. 

System Nine is a big prequel for System Ten. It combines aspects from previous systems and requires pretty precise movements in almost every level. Since you can take your time here though, there shouldn't be anything too difficult. As far as the boss level, it is a collection of all the types of obstacles you've gone through, and despite its size, it's not too difficult. There are basically five mini levels to complete to get to each of the five dots. The boss himself doesn't do anything special, just chases you, so just avoid him like usual. 

Upon completing all 20 levels and boss level IX, the achievement will unlock.