This achievement is fairly simple, as beating the game with lowest brightness settings really doesn't affect gameplay at all. In the main menu in the top right-hand corner, you will see a sun icon. Click this and set it to low. From here, just play through the game (main story and nightmare levels) to unlock the achievement. It's a simple, very linear sidescroller so not much direction is needed.
One very helpful tip however, as you play through the game, you'll eventually come to the later levels which include moving obstacles. These obstacles include saws, falling rocks, flying death squares, etc. Essentially, what's gonna happen is as these obstacles make their way towards you, pause the game. When you pause the game, Nep2no comes to a halt. The thing that sets this game apart from others is the fact that the obstacles don't stop moving. It will come right up to you, merge through you like you don't even exist and then it will continue on its way. That's when you resume it and continue on your way. This glitch is EXTREMELY easy to pull off, and it will save you hours. Good luck.