Step into the Light - Clear all missions in Heaven or Hell Mode.
Heaven or Hell Mode is an unique difficulty in which you die in one hit, but the same goes for enemies and bosses.
Although this mode sounds difficulty, it is honestly just a matter of spamming the Shoot button a lot and just progressing through the game. More or Less you can class this as a speed-run.
The only part you can't do this is in mission 11 and 20 as you have to break the shield surrounding The Savior first, but that shouldn't be a problem.
Good luck and have fun!