For all of you who are unfamiliar with Soul Partners and Deathscythes, I will explain them here. Ahem. SO, your Soul Partner must collect souls to become a Deathscythe, the strongest of weapons and the best of the best with exclusive power stronger than any normal weapon. Deathscythe is just a name, mind you, you do not actually have to be a scythe.
So, to BECOME a Deathscythe, a Weapon Technician and their Soul Partner must collect 99 souls and 1 Witch soul, then be approved by a Kage to become a Death Scythe. There is a catch, however. You MUST, and I say again, you MUST collect 99 souls FIRST and then the FINAL SOUL MUST BE THAT OF A WITCH, or else all of your souls will be confiscated and you will start all over from 0. Confiscation of souls can happen for other things, too, depending on the severity.
There are also special souls and special cases that will allow you a shortcut to getting souls, such as the souls of Ninja (Anbu or Higher) are worth something between 50-99 normal souls, but I doubt anyone under Anbu will be killing an Anbu. Some really strong Commoners or Antagonists may have Strong Souls worth between 50-99 normal ones, too, depending on their power. Killing a KISHIN, however, will instantly give you all the power you need to become a Deathscythe, so not only will you not have to kill 99 normal things, but you won't have to kill a witch either... Good luck killing a Kishin, though.
Where to find souls, you ask? Well, that's where portals and missions come in. You will have to take missions and go through portals into the other realms to fight off creatures that will provide to you their souls when you kill them (For example, killing a Yeti in the Hail Valley will give you One Soul.) Try to go to portals often, and remember that Sannin and Higher will RP your opponents. Killing special enemies like the Abominable Snowman in the Hail Valley will also get you a hefty amount of souls, but it is very powerful, so beware.
ALSO, PLEASE, FOR THE LOVE OF GOD, KEEP TRACK OF HOW MANY SOULS YOU'VE COLLECTED. PUT IT IN YOUR SIGNATURE OR SOMETHING, DO SOMETHING SO THAT YOU ALWAYS KNOW HOW MANY FUCKING SOULS YOU HAVE. I ALREADY KNOW THERE WILL BE PROBLEMS AND I WANT TO ADDRESS THEM RIGHT. FUCKING. NOW. SO DO IT.
Anyway, try to get as many souls as you can and good luck in becoming a Deathscythe!