I do think there is a bit of trap when unlocking souls. Your big source of SP is leveling up. Unless you grind really hard you get your first two legendary souls in a week or two, your third after a month or so? But fourth one takes ages, as your gain from levels virtually halts. So all in all I would not detour from Herc - even if you can't get her soul weapon straight away, you will be able to get it before your fourth legendary soul.