Hint: Skip Boss Rush
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This is basically the same method used in the rare item bug trick but for skipping the entire boss rush mode and still obtaining the item at the end of the mode.
If you are afraid of data corruption, make a copy of the game before you do this.
Make sure you have the Cinquedia weapon. Hammer will sell it to you if you don't have it. Equip Succubus bullet-type soul and Cinquedia once in boss rush mode.
Stand near the right wall of each exit to a boss room and use the Succubus attack (UP + ATTACK) and quicky cancel with special attack. Quickly attack three times and do another special attack. After that your should have teleported through the wall into the next boss room.
Repeat this over until you have passed all the bosses and reached the last room where the item awaits you and the boss rush mode ends.
Depending on how quick you have beat the boss rush mode will depend on what item you will recieve:
8 minutes or more, Potion
Under 8 min, Nunchuku
Under 7 min, Terror Bear
Under 6 min, Death Robe
Under 5 min, R.P.G. weapon
In the room with the water, just sink to the bottom right corner and perform the bug there. You can also equip the Enchant-soul: Draghignazzo to avoid being interrupted while trying to do this glitch.