Ako's Red Haori is required this trick. When you get to the port town of Sakai and the demons come out to play, equip Ako's Red Haori. Start killing demons with Issen or Block Issen. Clearing each area of demons should give at least three quarters of a full soul bar. The port area is most lucrative (one full bar every time because of the fish demons). The Issen is not necessary, but it is most effective. You can upgrade all of you present weapons to full with this trick, since one level 2 weapon/equip requires about ten bars to level up to maximum.