This method is done by heading from Rabanastre to Nalbina, having only the diamond armlet equipped, going up to the deck and doing either a 32 step method (You make Vaan run 32 steps with his left leg) or waiting about 20 seconds and opening the door. Immediately run back to the door to the deck, and go up to the top level and feel around for an invisible chest that might be there. Wait until the kid running around and going back to his dad repeats this four times. As soon as he stops on the fourth time, open the chest. The Seitengrat bow will be there most of the time if you completed these instructions correctly.
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