Here's what you can do. First create an aura based off of Devotion Aura, preferably the item one. Set the armor bonus fields to negative values of your choice. Make sure it targets enemies only. Now you change the duration of the aura to the duration of the spell you're making. The duration on Devotion Aura is how long the aura lingers on the unit after the host of it has left the area of effect. Once you have both spells created just simply make an event using Unit - Starts the effect of an ability. After that make the condition Ability being cast equal to (Your spell here.) For actions have it Unit - Add ability, adding the aura you just made. Use a wait function and remove it right after. It being removed so fast won't matter because of the aura's natural duration.
You can also use a wait function to time the starfall effect and the aura effect so they are synchronized.
The only problem with this is that if there is an enemy with a positive or another Devotion Aura, the positive one will take effect and the negative one will not work, messing up this ability. If that's the case you could use dummy casters.
I hope this helped.