I hope this video helps a bit with positioning for the proxy! There's a seam in the floor, you want to get low enough and charge jump through that. In other words, tap x while holding square in the right spot. If you're anywhere past that seam, you won't be able to make it. The trick is positioning yourself so that you can charge jump through correctly. Tap or mash x to help get lower in the floor.

a bit of a rushed video, but hopefully this explains a little better

I accidentally found this while trying to do the regular waterfall proxy in epsxe. It seems like it's probably easier and faster to pull off? I missed all the jumps, but it was mostly because the square-equivalent button on my controller is jammed. Just wanted to make sure it's clear that it's obviously 100% possible :p

I'll try to record a cleaner take tomorrow, but yeah, for now I'll leave this here.