The area walled off north of the Source Temple is not accessible, and even if you could get there, your characters would not be able to move.
There are a bunch of plants partly obscuring the false wall in the archway that you need perception to detect, at a location
x:126 y:163, as shown under the mini-map with the keyboard UI, and on the map with the controller UI.
It is near a chest, across from the path that leads down and eventually to the alchemist. Earth summons have good perception, but I'm not sure if the spider would be able spot it.