Ooops, forgot about that part. Well, for that you will need triggering. But its MUI and its only GUI. So yep, its good enough.
Objects you need: A dummy AoE spell to give to the caster, a faerie fire spell which costs no mana, and has the effect you desire for the Faerie Fire which affects the units, and a dummy unit. Make sure to assign the Faerie Fire spell to the dummy unit and the AoEDummy to the original caster.
Trigger (note that x is the AoE of the dummy spell)
Event - An Unit Starts The Effect of an Ability
Condition - Ability Being Cast equal to AoESpell
Actions - Unit Group - Pick every unit in (Units within x of (Target point of ability being cast) matching (((Matching unit) belongs to an enemy of (Owner of (Triggering unit))) Equal to True)) and do (Actions)
Loop - Actions
Unit - Create 1 Dummy for (Owner of (Triggering unit)) at (Position of (Picked unit)) facing Default building facing degrees
Unit - Add a 1.00 second Generic expiration timer to (Last created unit)
Unit - Order (Last created unit) to Night Elf Druid Of The Talon - Faerie Fire (Picked unit)
That is all and I assure you that its 100% multiinstance. It will work for more than one unit for the SAME PLAYER casting AT THE SAME TIME. That is because you didn't use any global variables. Cheers!
~Daelin