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.
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory