This refuses to work

Allright... in my map with mad custom abilities i'm having several aoe spells that cause different effects. Right now im working on a blizzard that when units are hit by it, they get lightning shield cast on them. Ive tried this two ways.

testing
Events
Unit - A unit Begins casting an ability
Conditions
(Ability being cast) Equal to Rain of Fire
Actions
Wait 3.00 seconds
Unit Group - Pick every unit in (Units in (Playable map area) matching (((Triggering unit) has buff Rain of Fire) Equal to True)) and do (Unit - Order Priest 0113 <gen> to Orc Shaman - Lightning Shield (Picked unit))

testing2
Events
Unit - A unit Starts the effect of an ability
Conditions
((Triggering unit) has buff Rain of Fire) Equal to True
Actions
Unit - Order Priest 0113 <gen> to Orc Shaman - Lightning Shield (Triggering unit)

the problem is definitely not in the casting of lightning shield, as i have made sure that the caster has no cast backswing and that he has the spell and it has no cooldown. How can this be done?

First off, make sure the unit has enough mana to cast the spell, or make the spell cost 0 mana. This may sound dumb, but also make sure you don't forget to spawn the dummy caster. Make sure the spell range is sufficient. Also, instead of Pick every unit in (Units in (Playable map area) matching (((Triggering unit) has buff Rain of Fire) -- try Pick every unit in (Units in (Playable map area) matching (((Matching unit) has buff Rain of Fire).

Hope it works out for you.