Comments posted to our Dark Souls 3 Wiki
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NOT weak to dark. Tested with gold, pale, and human resin. All three did almost identical damage. Either that, or is weak to all three.
Scratch that "weak to all three" part. Resins only added like 20-30 more damage. So yeah, don't use resins on this boss.
It is possible for phantoms to land a plunging attack in the beginning of the fight. However, it only does a charged heavy attack's worth of damage, which is disappointing considering you fell like a million feet from the sky
Take the plunge. They won't die.