Hm, perhaps this helps (event -> finishes casting... / condition -> ability being...)
Actions
Unit Group - Pick every unit in (Units within 512.00 of (Position of (Casting unit)) matching (((Owner of (Picked unit)) controller) Equal To Hostile)) and do (Actions)
Loop - Actions
Unit - Set life of (Picked unit) to ((Life of (Picked unit)) - 150.00)
I made the whole trigger, but for your question only this should be important:
Actions
Unit - Set life of (Picked unit) to ((Life of (Picked unit)) - 150.00)
This removes 150 hp (from enemy units (if using the trigger above, didn't test it))