Oh yeah, that part. I got stuck there too the first time. What you have to do is after seeing the guy on the switch, you backtrack to the room down below. A bunch of those guys that pop out of the ground will greet you again. After hitting up the save point, continue to backtrack a bit more until you reach a route that takes you over a bridge and to a new area. You'll get the lightning magic in the end, and that will let you kill the guy holding the switch.