I had originally thought she did it in a similar way to how Maple defeated fake Sally: by flooding the room with water and waiting for fake Maple to drown (Sally has Diving X, Maple has no Diving skill).
However, the skills that Sally had acquired during that battle seem to tell a different story:
- Cyclone Cutter;
- Combo Blade;
- Martial Arts I -> V;
- Wind Magic II -> III;
- Leap I -> III;
It is likely that some or even all of the above are red herrings. I'm not sure how to defeat a Maple in an enclosed space with the above. Suffocation probably wouldn't work with Wind Magic at such a low level, and the room isn't so tall that fall damage is a viable option.
Unless someone provides a viable alternative, I'll headcannon the drowning version of events.