Defeat all ten bosses that appear between 0m-9,999m. You must destroy all ten, rather than jumping up and avoiding the boss battles. With all bosses beaten, you will unlock the final battle with the Crimson King. Instead of attacking him, shoot your rainbows quickly and walk up them. This will lead you off the top of the screen. The boss will continue doing what he does while you keep climbing. Make sure to move left and right every once and awhile, as you will hit the edges. Repeat this until you reach 10,500m. As your falling back down, you should also get Questionable Talent. Here is a video showing this method. Notice the character is off screen but the meters are still going up. You can also reach 10,500m by beating the Crimson King and then chasing after Holly.
thanks to Todaysforgotten for the video