After you can enter the Ancient Cave, buy yourself a suit of Bronze Armor and the best weapon that you can buy at the moment, kill off your party members and get have the main character train as a Swordsman. Get him to level 10 and enter the Ancient Cave, if you can defeat the boss on floor 9, you'll get 6000 experience, make sure you have a providence because the monster on floor 12 will be able to do 80+ damage to you. I was level 25 and it was doing 97 damage to me. Repeat this process to gain levels very quickly. It's also a good way to master classes.