After finishing Border Forest, go back to it from the east side (the one you exited from; do not bring any monsters). Rope your way across the pit, go into the opening in the trees at the top,t hen through another opening at the right. You will find four rocks surrounding a bush. Break a rock, then cut the bush to fall down into a room with Green Cores. These monsters give 2525 experience points each and appear in groups of two or three. They have low HP, but very high defense. First, fight a few battles and kill all your party members except for one. Have him kill as many Cores as he can before they run away. Use Chance Hit, Virus, or any other group attack spells except for Frost. For maximum effect, make sure the character has an attack power over 120 and has learned the Rogue skill Rapid Fire. This allows you to take out all the Cores in one hit. When the room is cleared, exit and reenter to find the Cores again. When a character has mastered all the jobs, revive another character and kill the first. Then, repeat the process. This is the fastest and easiest way to master all the jobs and will make clearing the Ancient Cave a breeze for Eldin.