I noticed something, if you cast rain of fire on trees, it selects them as targetable but does not destroy them... I checked the "targets allowed" in the skill itself to find it empty.
Well then back to the original question, how do I make rain of fire destroy trees?
Uhm I think that the best way is to trigger it by using a JASS/GUI function that kills destructibles (such as trees). If you are using GUI, here is an example:
Trigger:
Rain of Fire
Events
Unit - A unit starts the effect of an ability
Conditions
(Ability being cast) Equal to Rain of Fire
Actions
Wait some time to match the fire animation
Destructible - Pick every destructible within 200.00 of (Target point of ability being cast) and do (Actions)
Well, just to be precise, if you want to match exactly the Rain of Fire animation, the wait time should be 0.80 seconds.
I made for you a simple map. Cheers.