Walk forward and pick up the rune. Don't bother attacking the Chthon, you can't hurt it Instead, run down either passageway and grab the 100 health bonus at the end, take the lift up. Now run in a circle around the upper platform pressing on all three floor buttons. This will give the Chthon an electric shock if you can perform it quick enough. Do this three times, it dead. A new platform will apear over the lava, drop onto it, and exit this episode.