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octopuslines:
The point is you can usually "fight" elemental type beings with other elements, too, except their own type or the type they are strong against.
So for example usually you can fight "Fire" with "Water" but also with "Earth" and "Wind" might be possible depending on the setting as in some settings "Wind" strengthens "Fire" and as such cannot harm it.
Therefore in this situation you can usually fight "Darkness" with "Fire" as "Fire" is close to "Light" and also emits light.
But the setting here is that the "dark matter being" is immune to everything except the powers of attribute "Light" itself while all other powers are negated.