After you reach the top of the tower in chapter eighteen (entitled The Core), save your game. Then, use the black floating panel next to the save point, which will refill your life and Ninpo. Equip your "Art of the Fire Wheels" spell, and go to the top of the ramp you just climbed. Use the Ninpo and simply stand there while the fish jump up and die on your fire shield, one after the other. You can repeat this as needed for maximum yellow orbs.