Leviathan.Celebrindal said:
»enemies in Sheol C do not specifically have a weakness the way their counterparts outside do. They have a resistance. Think of this as damage types to avoid, not a damage type that's best. One will suck, the others will be normal.
They have both weaknesses of their outer-world counterparts, in addition to any Odyssey weaknesses/resistances. For example, skeletons resist Slashing and Piercing normally, but in Odyssey, they get an
additional 50% slashing resistance. Piercing is actually stronger inside Odyssey vs skeletons because the native slashing resistance to skeletons + Odyssey resistance is higher than the flat Piercing Resistance. But Piercing is much weaker on skeletons outside of Odyssey, because it doesn't suffer than same slashing resistance penalty.
I physically do far more damage with Shining One on any Skeleton than I ever can with Masamune inside Odyssey. A similar case applies to ghosts as well.
Another example are birds. Birds are weak to piercing in general, but have a blunt resistance inside Odyssey. No penalty to slashing. So piercing weapons actually does more dmg to birds inside Odyssey than Slashing weapons because of the native weakness and from not suffering from piercing penalty.
To OP: Most of the enemy weaknesses are either latent/common knowledge, or buried in the old ffxiclopedia pages. You can go visit each family manually and see the weapon/elemental weakness/resistance, but I don't believe there is a master list. It would take a while to compile, and there's certain nuances between families that could be special exceptions, depending on the mob type.