All you need is make a spell from fearie fire or acid bomb bcoz they reduce enemie's armor. Make it 10 level for example. Each level has -0.5 increment to armor.
Then, create dummy unit and give it this ability.
Make the main spell based on starfall and you can make trigger so:
Events:
Every 1 second
Condition:
IsChannel = true
Actions:
Set counter = counter + 1
Pick nearby enemy units around the caster:
Create Dummy unit
Set Level of NegativeArmor to counter
Add 1 second expiration timer to last created unit
Order last created unit to cast NegativeArmor to picked unit
All the bold typed are the variables that you must created.
Then, make another trigger that set the IsChannel = true if the caster cast the ability and IsChannel = false if the caster stop channeling. Don't forget to set counter=0 when the caster start channeling.
JASS is script language that behind most of all warcraft 3 stuffs, including trigger. So, trigger is actually JASS, but with friendly GUI so it make people easier to use them.