This takes place very close to the start of the Mario Battle in the Genocide Route of the Undertoad AU. This is not official, just how I think at least part of it will go.
A white comet passed through the battle box. The toad's max HP and normal HP was dropped to 1.
"Don't be-a hasty, this is going to need to be a Perfect Run," Mario said. "No damage taken whatsoever. One hit and you lose a life."
"You're going into Culmina Crater with 0 lives."
The soul turned blue, and the battle box split into 8 pieces. The pieces shrunk, then arranged in a line with gaps between them. A white Charging Chuck appeared at the far end, and charged toward the soul. The soul jumped over the charging chuck to the other side. The chuck charged again. The soul jumped back. The chuck disappeared, and the platforms turned yellow. Firebars appeared above and below the platforms. There was a green circle on the far side of the yellow platforms. The playform the soul was on started falling. The soul jumped to the next one. It started falling too. The soul continued making its way over the platforms while dodging the firebars. The soul touched the green circle. The attack ended.
* It's World Crown-Crown.
Sorry, but I came up with this idea, and NEEDED to write it. Sorry it's so short.
Also, my explanation for why there is a Charging Chuck attack is that Mario can capture them in Odyssey.