So as bddatabase is dead now here is the picture of "where to go" in order to acquire this specific knowledge. Go in the caphras cave at west at florin and look for this specific spot in the cave (2 picture of 2 different view of the spot).
https://imgur.com/a/RxIXdah
You can either attract the monstre by puting a LOW LEVEL horse at the botom of those little platform OR ask a friend to help you with an AFK low level character wearing narchillan, he must stay at the bottom of those little platform (the character will not take any dammage as narchillan is too powerfull against those mobs). The high level character must use nice AoE skill in order to blast the area of the low level character and those spirit will get kill before they explode. You may notice that you can acquire some blue flask when successfully killing them, if you do then keep up because its just a mater of time!

You will kill those black spirit by wave of 3, they do respawn a bit longer than the other monster, approximatly 3 min total.
Use weenie/looney elixir ; pets; simple cron food; lightstone combo ; and pearl outfit if available.
It took us ~15 min to get an S grade on 2 different character using this strategy (~120% more knowledge acquisition and ~25% higher grade knowledge).

This still works as of 12/31/2022

You can buy a low-level horse in Florin at the stable master. You can equip it with DP horse gear (Barding, Stirrups), call it down into the cave with you, and park it right ontop of where the reddit thread pictures. This is down at the bottom of the cave, just above the bottom most excavation by a witch npc. Auto-attack while the Roaming Black Spirits try to attack your horse and you should get the knowledge regardless of your character level. I tried this on an Awakened Warrior Lv. 60 and got "S" from "C" in less than 5min, without the horse I spent several overnight stays with no knowledge gains at "C".

This is the by far stupidest Knowledge to gain. You have to kill black spirits who appear for a span of 1 second, till they suicide. You need to hit them to gain knowledge. See above post for more information and good spots. I as an awakened Warrior went into that cave spamming Left Click + S, "mowing" the whole cave. Every Black Spirit appeared instantly died.

Everytime you hear a specific sound (ghost flying through air kinda thing) hit an aoe attack

Stand in location where on minimap it should be, and spam AOE skill. When this mob attack you, there will be vulnerability window in about 0.5 sec and you must to hit it just in time.

And you can use the "red ball" on the female wizard who create an aggro for make them appears and attack. It could help.

As suggested, you need to spam autoattacks where the reddit link says so, but if you and your horse are of a high lvl, the black spirit won't aggro on neither of you, i needed to buy a low lvl horse and put it where the screenshots and video shows, and got it in less than 10 minutes... i did it on a lvl 62 corsair only spamming LMB while at it, i hope it helps to get your S rank guys

Hi Ppl, I've been trying to fing the knowledge and the only possible way it was with a horse lvl 1 in this location. First you kill the mobs around so you delete every sound around. the mob before atacking make a huge sound so you will know is coming. it look like you have to kill him when he appear, he is really fast so hit quickly.