This post originally started as an endeavor to answer questions posted in this topic (http://nifflas.lpchip.nl/index.php?topic=2487.0) and this topic (http://nifflas.lpchip.nl/index.php?topic=572.0). Seeing how there wasn't a 2468 challenge specific topic, however, and since the video links in either topic appear to be dead, I thought it best to start a new thread.
Anyone who's been around Within a Deep Forest (WaDF) likely has some recollection of the 2468 challenge, but it appears as if there isn't actually a formal thread for it. The challenge, at a most fundamental level, is to beat the game without acquiring ball materials 2, 4, 6, or 8 (:a2::a4::a6::a8:).
In a typical playthrough, materials 2, 4, 6, and 8 are utilized primarily for their fast motion and subsequent ability to reach high places. In the 2468 challenge, one must substitute material 5, the glass ball (:a5:) to reach some of these places instead. The challenge lies in not shattering the ball in doing so. Some ledges are too high to reach without hitting the ground at high speeds, and thus the corner glitch must be used. The corner glitch involves an immunity from breakage at high speed if the glass ball collides with both the horizontal and vertical components of a corner simultaneously. The difficulty in this challenge lies in being able to perform said glitch consistently.
The current most accepted order (and also the speedrun route) is to obtain the materials 3, 5, 9, 0, then 7. It has been theorized that obtaining 7 before 9 or 0 is also possible, but it would require entering Crystal Path with glass ball, which means performing the corner glitch at high speeds and against wind.
It is not possible to make the jump from the ground to the lowest blocks of the Shinock Reign tower with only the starting ball. The challenge bypasses this obstacle by instead jumping from the top of the mountain and sailing down to the tower.
The glass ball and corner glitch are required to enter and beat the Shadowlands.
The 0 ball (:a0:) is typically used to enter Crystal Path.
My walkthrough of the way I personally learned to complete the challenge (https://www.youtube.com/watch?v=f_a1QBXrWUg)
Insane speedrun which shows of what glass ball is capable of way better than I ever could (also the current world record) (https://www.youtube.com/watch?v=Sf5-FdoZRSM)