It's a mirror puzzle. Complete it within the time limit. You're near the end of the game too.
I've finished the game on normal, now hunting the secrets. I've pulled the first secret switch (fairly certain there's two more) and found the Garden Key, but nowhere to use it. I've got far enough as a virgin to check certain things. One of the screenshots above just confirmed something for me about the first switch.
Have not found the Kunoichi yet (I can guess which game area), or the other two secret switches. According to comment above, there's no 3rd switch, so I might hunt for the 2nd without spoiling myself. It has to be in the Kunoichi area. I wonder what the Garden Key is for, seems like it might be for the unimplemented 3rd switch, maybe the last W.I.P. room.
edit: found and completed Kunoichi challenge now.