Shortly after your time in the first world, you will start to see some crabs running around with treasure chests on their back. These are the infamous "gold crabs" that are mentioned in the description. They have a scope of walking about two or three spaces for every turn, but easily snag themselves on corners. If possible, have Lilliel aid you in backing one into a corner and start hitting at it (don't waste SP on it). Your chance of hitting it will be rather low compared to other creatures, but it only has 4 HP. Every time I've killed one, it's given me a decent amount of gold (compared to the worlds themselves) and at least one level up every time, so it might be in your interest to seek these little beasties out any time you come across one.