Leveling custom spells

I'm trying to create a unit with a custom ability and forcibly level it depending on what level its dummy spell component is. For example, I have a custom spell that's level X, and when its cast, it summons 4 wisps that each chain the target unit with a custom chain lightning spell. How, when I create the wisp, would level its custom chain spell to the level of the dummy spell?

You'd have to make a different version of the wisp for each level of the spell. Make 3 unit versions of chain lightning and give level 1 to the wisp that's summoned in the level 1 spell. Give level 2 to the level 2 wisp, and so on. The only way I can think of to do it.