You can get there without Luigi. Reveal all of the blocks with Tippi or Tiptron, and jump on them to make your way towards the last platform. Flip into 3D from there and go through the path. You should end up on the blocks on top of the pipe leading to the Flopside Pit of 100 Trials.
That's the thing! For whatever reason, I couldn't find the hole in the wall there! (and I was actually able to get up there without Tippi using
Dashell and the block behind the pipe, then found the blocks. I'll look again, though...
EDIT: Okay, I don't know why I didn't see the hole there, it was pretty obvious... happened to me in chapter 7 once too.