Follow these steps to reach the Mirrored Ninja Boss:

-On scene 9, enter the elevator (first door) and go up to the 9th floor.
-On the 9th floor go right and enter the first door you see, it will send you to the 7th floor.
-On the 7th floor go right and enter the second door you see, it will send you to the 3rd floor.
-On the 3rd floor go left and enter the first door you see, it will send you to the 2nd floor.
-On the 2nd floor go left and enter the first door you see, it will send you to the 13th floor.
-On the 13th floor go right to face the mirrored Ninja boss.