How do you do this (trigger question)

I'm working on some trigger-based abilities. Is there anyway you can say something like...

Event
Unit - Unit begins casting a spell

Condition
Spell Being cast is "..."

Action
Set Unit being hit by spell = TargetUnit

I want to have the unit that the spell is being cast ON to have other effects happen to him after the spell is cast, but I can't find any Action that seems to do that. The closest I can find is, "Event Response - Casting Unit." Is there anything like "Event Response - Unit being effected by spell?"

Does that make sense? :bgrun:

Depending on the kind of the Ability, you could try to pick all targeted units and then do something with Picked Unit.

That's what I have been doing for some of the other spells, but I can't with this one. I need it to only affect one single unit.