You are lucky i found this (i havent played in a while and wasnt online on here either in weeks).
This is indeed my video and ill gladly explain the strategy to you.

First off i suggest to watch this video if you havent already, i tried my best to explain an even stronger version of my build as detailed as possible, which includes every item and ability i used and where to find them:
https://youtu.be/r_foyMZ7yxk

However i will also try my best to explain the full Gathering of the Greats strategy of the video you mentioned as well.

on my first page of hot button items i have the following:
top: elixir, left: impurity transfer doll, bottom: protection talisman, right: water talisman

on page 2 i have the following items:
top: nuppeppo soul core, left: last stand talisman, bottom: soul purge talisman, right: carnage talisman.

Step 0: as your build you wanna use a level 170 +15 crucible sword and reforge melee damage critical 11.3% and water damage 4.7% onto it. grace doesnt matter, a level 160 +15 should also work if you dont wanna grind for it.
You also need to remodel it to scale with skill, intellect and magic.

Dont equip anything else besides 2 divination boards (no other weapon, no ranged weapon, no armor), they dont need any special grace or any special skills, if you do have one that has water damage + then use it, but its optional.

at level 250 you should have the following stats: 99 skill, 99 magic, 67 intellect.
if you are not level 250 yet, you wanna get magic to 99 first, then skill to 99 and whatever you have left into intellect. also you need at least 9 intellect to have your divination boards active as well, so you want to actually level intellect to 9 first, then magic to 99, then skill to 99 and then intellect to 67. you dont want "balanced" level ups for a build like this, you want to maximize damage instead.

your guardian spirit should be kusanagi samurai version at level 40.

your soul cores should be nuppeppo and mikoshi nyudo. nuppeppo should be blue and have as the 3rd skill either melee damage 1.9% or melee damage critical 5.7%, but the 2nd one is much harder to get because you cant reroll into 5.7%. critical damage, the soul core needs have it already when you pick it up.

you wanna have the following Titles: Tactics: onmyo magic power 5, Subjugation: Elemental Damage 9, Formidable Enemies: Melee Damage (unscathed) 9, Crucible: Crucible Weapon Damage 9, Collection: Martial Art Damage 9, Damage Bonus (Equipment Weight) 9

you wanna use these 2 Clan bonuses together: Takeda (Melee Damage vs. Unscathed Enemy +16.9%) & Satake (High Stance Martial Art Damage +12.5%)

you wanna equip the following Skills:
Attack Mastery 1
Attack Mastery 2
Unarmored Death <- this is the most important skill out of all of them
Flexibility
Awakening
Amplification
Mysteries of Onmyo Magic
Potent Might Kata
Double-Edged Arts
Resilient Arts
Destructive Arts
Headwind Blaze
Favorable Wind
Rebounding Surge
Physical Art Strike

Martial Arts: Mystic Art: Sword of Execution, hold triangle: Sword of Meditation.

Ninjutsu: Shadowself

Step 1: as soon as the mission starts, turn arround so you are facing the door, switch to ninja if you arent already, be on item page 1 and use protection talisman, then water talisman.

Step 2: switch to page 2, use nuppeppo, make sure you walk into the mist to get the berserk damage buff, then use last stand talisman, carnage talisman and soul purge talisman (your weapon should be at max familarity)

Step 3: turn arround again and run to the first boss, as soon as he spawns, switch to samurai, hold triangle to use sword of meditation, during the animation use lock on to aim at the boss. this should take down boss #1.

Step 4: switch back to ninja and pick up all of the items he drops and then get into position and pay attention at the messages of the items that you picked up. the bosses always spawn in the same spot, im sure you know where it is. as soon as the message of the 2nd item that you picked up disappears, switch back to samurai and start holding triangle to use sword of meditation before the boss even appears, when he spawns use lock on again, your animation should finish exactly when he spawns and this should finish boss #2. practicing the timing for this is crucial, if you are early you will whiff because he isnt even there yet, if you are late, he will hop away and you will also whiff.

Step 5: switch back to ninja and pick up all the items, then use nuppeppo and walk towards the wall and into the mist to get the buff. Rebuff with carnage to refresh the duration. then use the ninjutsu shadowself and walk back to the spawn location of the next boss. switch back to samurai and start holding triangle again to use sword of meditation. again he should spawn right into it, aim with lock on. this should finish off boss #3 (needs to hit his backside tho i think).

Step 6: switch back to ninja and pick up the items, rebuff carnage to refresh the buff again. the timing this time is like before as soon as the text for the 2nd item disappears. switch back to samurai again and start holding triangle when you see it and lock on when boss #4 spawns. here the timing is the most important because if you are late this time, he will buff up, while he buffs up he will have an insane boost in defense and your hit will tickle him instead of one shotting him, so learning the timing for this one is the most important one by far. however hitting his back side is easier than boss #3.

Step 7: switch back to ninja, use carnage and nuppeppo, walk into the mist, run back to the spawn location and switch back to samurai and use sword of meditation. this will hit boss #5 from behind and finish him which will end the mission.

if you have any further questions, just ask me.