Here is a nice one! I would be surprised no one found it before, but just in case.
In Naga City, the Hooded Figure simply sits on top of the Nagarena stairs, but the stairs are still here. Since the Hooded Figure is Pacifist, there is no way to make him budge... except with Mass09's Ledger.
With it, you can access the Nagarena without triggering the dialog, which means that the entrance stairs from the Arena are still here...
You can then do the whole Arena fight and still use blackspace from the top level.
Isn't that nice?