ATTENTION!!

***This article contains spoilers, so it is suggested to read it after finishing its current version! ***

Inside this game, "Soul Weapon" is a special weapon that is usually well hidden inside the stages of each Hololive Member. Lore-wise, the existence of these weapons is explained like this: When the girls were imprisoned inside crystals to become the heart of a small dimension, a portion of her "soul" was separated from the control of the bad guys and became a weapon, so the Myth girls would find it and use it to help save everyone. Using Crystals (which is basically the currency of the game) the weapon can evolve into a stronger form, and the correspondent idol changes into her stronger Class.  Now, if you have problems finding all of these you could just finish the game, and winning in the Extra Mode you would get all the weapons you lack. Otherwise, you may consult this guide here to make your journey easier! Note that the Myth girls' weapons are not Soul Weapons, those are weapons they had already with them, to begin with (so you will get those as the game proceeds in unavoidable events, so you will not miss them).  Also, Coco is the only one that doesn't have a weapon because, again, she was not "separated" from her soul considering she arrived in this weird world after it manifested. 

The first half of the Soul Weapons were listed here, so you may check that one as well:  Soul Weapons Guide Part 1

1. Noel and Flare: One of the less intuitive equipment to find. In the second section of this Stage, you will notice a door that can be unlocked using a key. Go back, since in the same section near the beginning on the low side. there is a skeleton, The skeleton has the key (this is a Sans reference since Flare loves him so much by the way!) In the room, you will find the Sacred Elven Bow and  Danchou's Mace. Upgrading these you will obtain the Couple Killer Bow and Shirogane Mace.
   For the accessories, after defeating Noel and Flare Alter version, search for the other skeleton, then go right, to the building above the statue. There is a stone door that can be opened with that second key you just found. There, you will get the Kinstuba Charm and the Shirogane's Armor.
2. Rushia: Again, you will need a key to access the room where here equipment lies. The key is found on a chest, right to a tombstone with a cross more or less at the center of this Stage. The room with the items is instead located on the top right side of the map. Her weapon is the Rod of Death, which evolves into Lucifer's Scythe.
3. Towa: In this difficult labyrinth, the room can be found on the lowest side, towards the center. You will need to decipher a riddle, but I'm pretty sure you don't need my help to answer that one, trust me! You will find the Evil Lance, which evolves into the Eternal Darkness.
4. Kanata: The other labyrinth that even I as its developer keep getting lost during tests(草). The items can be found on the top right side of the map. Kanata's weapon is the Angel Beats Them All!, which evolves into the Valkyrie Lance.
5. Suisei: In this pretty simple Stage, you find Suisei's accessory on the top left side of the map, on a "mimic" chest that is a forced battle. So, you get the Suichan's Gloves, while defeating Suisei's Alter version gives you the Golden Axe, which evolves into the Starlight Axe.
6. Miko: Her items are found inside a room on the top left side of the map. But beware! There is someone guarding those items!! Miko's weapon is the Gohei Whip, which evolves into the Elite Gohei.
7. Mio: Mio's items are hidden inside the first house you see after defeating the mid-boss of the Stage. Her weapon is the Tarot Deck, which evolves into the Tarot Deck Deluxe.
8. Fubuki: After being able to defeat Mio Alter, you will have to deal with a "mirage labyrinth", in which you need to proceed in a certain order to get to the deepest part of that forest. To save you some time, the path order is bottom, right, bottom. When you see the Green Crystal, go towards the path on the left side of that section, close to the road sign. There, you will find the Shirakami Katana, which evolves into the Shirakami Katana 改.
9. Haachama: After you defeat "Haachama" once, you will be transported to an area that is similar to another place, check the bottom part of the room where the blonde girls are standing and you will find her items. Haachama's weapon is the Haachama's Spatula, which evolves into the Kubikiri Knife.
10. Botan: The items are hidden in two walls with cracks on them. To have a sign of their presence, take with you a member of the 5th Gen. In any case, the Lion's Rifle is around the middle bottom side of the Stage, while her equipment, the Lioness Black Jacket is found on the top right side of the Stage. Botan's weapon evolves into the Lioness AK-47.
11. Ayame: Her stage is basically the same one in which you find Mio Alter the first time. But be careful this time: when you go up the stairs, after the torii, move towards the left and then up. You will find a path that allows you to change sections. There, you will find only her equipment, the Oni Mask. To obtain her weapons, you need to do the Class Upgrade for her, then you will get automatically her two swords, Rakshasa and Asura.
12. Roboco: At this Stage, defeating the Robosa will give you some batteries, needed to charge three doors on the whole stage. Then, moving to the bottom left side of the area, you will find a terminal and, a little above, a closed door. That is the Ganma Door. Putting the batteries into the door and then unlocking it, give you access to Roboco's items. Her weapon is the Laser Arm, which evolves into the Laser Arm Mk. II (this is more less a reference to the fact she keeps changing arms in Holo Grafitti).
13. Matsuri: One of the trickiest items to obtain: Matsuri Alter will jump into you occasionally through the whole Stage, and after three turns she will escape. If you defeat her before getting the items, you will lose the chance to get them, so avoid dealing too much damage to her until you find the items. These are found in the top right house with an entrance. Inside, you will find the Cheer Leader's Pom Pom, which will evolve into the A.P.EX Carbine.
14. Iofi: In the Stage where there is a tall tower in which you keep fighting Nun Nun and the Thirteen Knights of Sora, at the very top, Iofi's items can be found. Her weapon is the Pencil of Creation, which evolves into Pencil of World Creation.
15. Sora: The last Hololive Member. She has with herself her own equipment when you recruit her the first time: Heart of Hololive and Ankimo Amulet. Sora's weapon evolves into a stronger version of itself.