After you acquire the spell and return to the main room, use it to get across the room on the other side of the gargoyle. You will need to use the hardly visible ledges to cross. Hang off the wall ledge to escape nearing ghosts, and push down the slabs to make a bridge. Once this is done, you will have to fight the gargoyle. It is recommended that you get on the right staircase, halfway up. Then, use the spell on the last stone and the door will open.