This is actually very easy compared with the American version. Once you acquire the Wolf and Bat forms go back to the room with the rock tunnel in it. Go through from Left to Right as Wolf and Right to Left as Bat. This opens up a secret room (needed for 200.6%) and inside is the Jewel Sword. The difference between the versions is in the US version you don't get killed enemies to drop rings like they do in the JP version. In the JP version about 1/3 of every enemy (sometimes more I would get rings every enemy usually) will drop a ring. Most of these are the Zircons but sometimes you can get ANY ring. Before I finished the Inverted Castle I was able to sell all my rings for over 300K!!! The Jewel Sword if obtained as early as possible will be your strongest sword until you get to the Inverted Castle and acquire the Crissagraem! Good luck!