Maybe emotions are the key to beating these creatures? As cringy as it sounds, "love overcomes evil", "light, goodness, hope" etc banishes/obliterates demons and darkness and all that? Makes me think of Fairy Tail, Sailor Moon, I think Cardcaptors Sakura and a bunch of superhero kiddie western cartoons I'm sure too ofc.
Given Brooks powers revolve around emotions too with his music, manipulating emotions and senses, he might become a MVP of this arc too like WCI.
Lmao they said its Shanks but it could be Shamrock now too ofc?:
https://www.reddit.com/r/Piratefolk/comments/163ih8k/_/jy2nwfv
Point 6.1: „The Tragedy Of Immortality.“
https://www.reddit.com/r/OnePiece/comments/t59hmd
Something interesting I found randomly too btw: