Descend into the middle story, then make your way through the stairs in the middle. They appear
to lead nowhere, but there is an invisible floor. Jump to hide the invisible wall in the middle
of the invisible floor blocking your way. Proceed up the second set of stairs leading to the exit.
It appears blocked, but there is another invisible floor you can run on without the entity in the
middle of the room disintegrating the stairs. Use this to disintegrate the wall blocking your 
path to proceed through the rest of the area, which utilizes all the skills you will have learned
earlier in the game.