I've used cheat engine to teleport myself behind the rocks. Both sides have a set of stairs, and the stairs lead nowhere. You enter the stairs and the stairs point to the tile you're already standing on.
Placeholder stairs, with the Rocks blocking access to them until that stuff is ready. Makes sense.
Edit: God damn Mokou D: This is such a trap fight on Hard mode DX Challenge Level of 17. Even with several levels extra in Viole boosts, Mokou is faster then
by enough that she can sometimes get double turns. That 1100 HP regen she has, combined with having to Concentrate to keep attacking, meaning I can get her down to about 35% remaining before reaching a stalemate.
I regen HP faster then she can drain it, and even though I can drain hers faster then she can regen, I only get 7 hits like that before Concentration is needed, at which point she regens about as fast as I can drain her.
Trap fight, and I didn't save before going in, so if I close the game, I lose a good deal of progress x.x
Edit 2: After some smart use of the moves I had available, and a bit of luck, I managed to win o-o