Southperry's own Thief Flaunting thread =D

I see. How exactly to obtain a specific soul? Does it work for any monster?

max's blog talks about it in very good detail here.

all bosses drop soul pieces, but souls exist for only: cygnus, pink bean, von leon, ani, mu gong, balrog, hoblin king, soul of rock, dragon rider
i may have forgotten one or two, my memory isn't that great with this.

there's also good reason to keep it charged; the attack/magic attack of the weapon will increase as the soul gauge goes up.