I cannot find it. Things I have tried include (all attempts were after enemies were cleared):
- Running around the stone column and hitting it
- Breaking all the barrels and torches
- Breaking all the barrels then breaking all the torches with Icicle Edge
- Breaking all the torches with the Hammerball
- Running around the edges of the map trying to find some piece of wall to hit to trigger it
I give up. I'm really hoping that this isn't a case of just having to pixel hunt to get the exact right area.