You could use a damage detection system. Detect when spell damage is taken. If the target unit has the buff from your aura, deal extra damage. It would take more work because you would need to trigger all of your spell damage. But it would give you far greater control.
Dummy aura as you said with elunes grace works exactly the way you wanted it too, did you know if you use negative values you can get spells to heal enemy or allied units?
Just a idea if you thought of elemental damage as in fire vs fire.
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