Color Symphony 2

Simply finish the second stage of the Dark Forest bonus chapter to unlock this achievement.

I'm not sure why this stage has its own achievement. It is a very difficult stage, especially if you aren't good at wall jumping (you'll spend the entire stage on wall jumps), but definitely not the one I'd consider the hardest. Refer to the "Conqueror" achievement solution for a video walkthrough of all stages, including bonus chapters. Stage 2 of the Dark Forest starts at 15:26.

As I recommended in the Roadmap, you'll want to have a firm grasp on jumping with  because you'll need to continually change colors in mid air on this stage, between and during wall jumps. It will definitely take a ton of attempts to get the pattern down, so just keep trying until you can perfect it.