When starting, go to Witch Woods which is the 2nd map after the town. Go to any of the swords on the map to trigger fight, but don't trigger it yet. Instead, save in front of it and trigger the battle after. If you encounter the gold dragons, defeat them and you should get a decent amount of cash. If you don't get it, run and trigger it again. After defeating it, open your menu and standby to return to school, your view just changes to the school, then return to your party. The sword will respawn and you can battle over again. Repeat as long as you want.