Instructions
Place Arche Klein, the wizard, in your main party. Since Urchins can kill all your characters simply by touching them, you risk getting a game over screen unless you have Arche in your party. Because she does not walk but floats, the Urchins can't touch her, making it impossible for them to kill her and end your game.
Play until you reach the Morlia Mineshafts, where you will fight the "Bigfoot" enemies. These foes drop a item called "Ivory" when defeated, so collect these.
Take the Ivory to craftsman Gheeth, who is located on an island in the middle of the ocean. You can find it by traveling south from Freyland. In return for the ivory, you receive Magic Tablets, the only item capable of killing Urchins.
Open the battle menu when you encounter Urchins. Select the Magic Tablet from the item menu. Select the Urchin as the target for the item, and you will be able to kill it.