Something like this:
-It has much leaks but since you seem a newb, you can ignore that until you can make your spell work
Nova
Events
Unit - A unit Starts the effect of an ability
Conditions
(Ability being cast) Equal to dummy ability with no target ( like battle cry )
Actions
Unit Group - Pick every unit in (Units within 600.00 of (Position of (Triggering unit)) matching (((Triggering 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 (Casting unit)) at (Position of (triggering unit)) facing Default building facing degrees
Unit - Add dot spell (a single target spell that deals damage over time(dot)) to (Last created unit)
Unit - Set level of dot spell for (Last created unit) to (Level of dummy abilityg for (Triggering unit))
Unit - Order (Last created unit) to Orc Far Seer - Chain Lightning (Picked Unit)
-My dummy units have negative regen so they get removed