Instructions
From the level Flipside B2, use Luigi or Boomer to reach the secret switch.
Find the pathway underneath the Cafe while in 3D, in order to find the second switch, which will open the gate to the Flipside Pit of 100 Trials.
Win the Flipside Pit of 100 Trials by defeating the enemies contained within each of the 100 rooms, and by beating the final boss Wracktail.
Find the Flopside Pit of 100 Trials, which is located in the exact opposite location of the Flipside Pit of 100 Trials.
Beat the Flopside Pit of 100 Trials twice to obtain the Mario Card, which will allow you to use the 3D ability with no time limit.