Getting credit for lightning shield kills

I'm making a map where I thought it'd be cool to use lightning shield offensively. The hero casts lightning shield on another guy, and if the guy with the lightning on him kills another guy with his lightning, the original casting hero gets credit.

Only problem is "Event Response - Killing Unit" returns the unit with the lightning shield buff, not the caster of it. I can't think up a way to give the caster credit.

I could add units that have it cast on them to a unit group say, and then if the game gives them credit for a kill and they're in the unit group, give credit to the caster, but there's no way for me to tell when the kill is due to lightning shield or due to them just attacking :/

I thought about figuring out a way to determine if a unit dies with the lightning shield buff -- but the buff only appears on the unit with the actual shield, not the units it damages.

Any ideas?