Enter Mt Cornet from the southern western end(the clostest to Oreburgh) once entered go up and surf east, asfter that use rock climb then enter the stairs. Then just press on.
There is a section on the far north eastern part of Mt. Coronet that you get to. There is a villa type area which has a cave entrance to the right alongside it and right on the end there is a rock climbing surface. If you stand right in front of this there is another on the left side. \i remember missing this bit because I couldnt see it properly. You climb up that one and keep on following the path where it takes you. If youre on the right tracks you will find a cave entrance where you fight a line of Galactic grunts as you climb some spiral-like stairs in the cave. Hope that is some sort of help.