Nioh 2 - Soul Cores

You will need to collect 49 different Soul Cores to get Soul Searcher trophy. Most of them are easy to get the drop from Yokai's, but some are rare and you will have to hunt them. You can check them via Items in your menu and you can sort them with R3 button to get the same list as I will provide. 

To raise the rate of souls to drop, on Shrine use Kodama blessing "Yamabushi Blessing". There are some skills as well, but they are not needed, it should be quite easy to farm.

Gaki
Enki
Yoki
Gozuki
Mezuki
Ippon-Datara
Infernal Oni-bi
Kappa
Nurikabe (wall-type enemies, maybe the best level to farm them is "The Viper's Sanctum" as it is quite close to the start. You need to attack the wall and kill it to get the core eventually)
Mujina (enemies that attack when you open a chest - video guide coming soon. "The Two Faces of Hospitality" mission is the best to farm them since it is a 100% chance to get them when opening a chest behind a wall there. To get the soul core, you can't attack, wait and replicate the gesture they do after they do it. If you fail, try dying to respawn the enemy to try again)

Enenra
Nure-Onna
Aberrant Soldier
Toxic Slime (only exist in The Viper's Sanctum at the bottom of the pit - there are 4 of them there and a save point close by)
Yatsu-no-Kami
Yamanba
One-eyed Oni
Karasu Tengu
Waira
Karakasa Umbrella (The easiest level to farm them is "Abduction" sub mission in the Dawn region as it is the closest spawning point. Just go forward and right, down the stairs and kill the first wave of enemies. I had issues with getting the soul there, so instead I went to "A way out" mission. There, from the 3rd and last shrine, go forward and on the left behind the fence you will encounter this enemy. Since the shrine is close, maybe this is your best bet - it was mine for sure)
Lesser Umi-bozu
Skeleton Warrior
Kamaitachi
Koroka
Dweller
Rokurokubi
Onryoki
Mitsume Yazura
Biwa Boku-boku
Tatarimokke
Flying Bolt
Namahage
Tesso
Ubume
Onyudo
Gyuki
Wheelmonk
Kasha
Ryomen Sukuna
Daidara Bocchi
Shuten Doji
Magatsu Warrior
Lady Osakabe
Otakemaru
Kodama (you get that one from leaving some soul core to Sudama - pink Kodama's. I've farmed them few mins in The Viper's Sanctum as there are 2 of them close)

Scampuss (cat type allies that you can pet and they follow you. The easiest to get those is to enter "The Forest Veiled in Darkness" sub mission, find one close and wait until it leaves. It will eventualy leave its soul core. If not, use the Divine Branch Fragment to leave the area and repeat)
Maelstrom Oni-bi (The easiest level to farm them is "Abduction" sub mission in the Dawn region - just close to starting point you can find 2 lights out of which comes the blue Oni-bi that can drop this souls)
Thunderstorm Oni-bi (The easiest level to farm them is "Abduction" sub mission in the Dawn region - just close to starting point you can find 2 lights out of which comes the yellow Oni-bi that can drop this souls)
Mortal (Basicaly this is human soul but from boss-like humans in levels. Where i've got them the most was "The Two Faces of Hospitality" mission where you can beat 2 stronger type enemies which drop them - respawning enemies do not drop those)