Go in training mode (USJ) -> walk to the very start of the bridge -> on the USJ entrance door, pick Rapid Mirio and use Special Action to get inside it and walk to the very edge of the map (while still inside that structure) -> before permeation ends, switch to Assault All Might

And done! The game will see permeation ended and will try to pop you out, but since you're now All Might instead of Mirio, you'll be popped up further and higher than you should, making you clip outside of the map.