What Lies in the Multiverse

• Chapter: 5
• World: Dark (as opposed to the Light one)
• Requirement: stay on the "dark" world until the time limit expires
• Story-related? No; if missed on your first playthrough, you can use chapter select

It is very likely that you'll get this one by playing normally.

The "dark" world in this level has a permanent poisonous environment, so staying in it for more than a few seconds will make the Kid dead.

Therefore, from the moment you gain the ability of switching worlds onwards, simply switch by using and stand still until the time counter runs out.