Video:

Psycho Waluigi Boss Skip in world 4
I damaged boost into the wall and fly down out of the boss room.
I found this accidentally during my casual play through and didn't see it in the WR video so I figured I'd post it.
It seems to be pretty consistent.

Ooooh, very nice, I'll have to try that out myself. I dislike the expanded health strat and this should be so much better.