I think CL 7 is ok. A party with access to move earth or greater dispel shouldn't have to struggle too much to get her out of the rock, and that CL is enough for her SLAs, too, I think.
De-advancing a stone giant would give +9 natural armor, but maybe that's too high. +7 like a thoqqua? Anyone have access to galeb duhr or some similar earthy critter? (Xorn have really high natural armor, just as an aside!)