After going through the first set of double doors where the Oni Army is fighting the Genma, before using the Firefly to go up the short cliff, have the Genma and and Oni Army men on the wall of the short cliff and you as close as possible to the doors on the other side of the field. The supply of Genma infantry are infinite and Oni Army can take care of them by themselves. While they are fighting and nowhere near you, tape down Circle and allow the game to run. The souls will come to you, and you can get as many as desired without interruptions from the Genma. For a slightly faster collection, have Ako's Blue Vest equipped. Note: The process of getting a lot of bars filled of souls will take a long time. Also, when the soul count reaches 9,999, release Circle. This gives you an additional bar (or perhaps more) filled automatically. Then, tape Circle again. Given enough time you can have 99 bars worth of souls.
Note: This trick only works for Samanosuke. Once you obtain the Ten-Point Slash Training Scroll, play under the beginner difficulty setting multiple times. Each time you finish, you should come away with one bar, or close to it. Do this until you have the desired amount of souls, or until you max all your equipment.