Ok I get it.
Here it goes......
Event :
A unit starts casting an ability
Condition :
Ability being cast equal to Rain of Fire -> (Base this skill in Channel, I'll explaing why later).
Unit type of Casting unit equal to Whateverdummy
Action :
For Each Integer A, 1-10, give multiple actions.
Loop
-Create 1 dummyone on position of the Casting Unit facing (facing of casting unit).
-Order (Last created unit) to Special Doom Guard - Rain of Fire in Position of Casting unit.
I recommend Channel for a dummy spell like this. Turn the UI key on on the object editor as well as make the target instant. You have no choice but to edit the Rain of Fire (Neutral Hostile 2) if you want to modify the whatsoever of your rain of fire.
If u want to make special effects and cast it after 5 seconds, just put in the actions the effects and put a Wait action for 5 seconds.
Voila! It's finished!
If u want to see the actual trigger, download my map. It's a good thing I have a ready map for this ^^. Find the trigger for meteor clash, if u want you could also check for the Ice dragon dive.