Go download Damage Engine
here. Follow the instructions from the description to implement the system in your map correctly. Once you've done, Create a new trigger. Put "Game - DamageModifierEvent becomes Equal to 1.00" as an event. The condition should be something like "(Level of YourAbility for (DamageEventSource)) Greater than 0", this will check if the unit that causing damage has the ability. You can alter the damage that the caster dealt using "DamageEventAmount" variable.
I find it hard to explain it, it's much better if you just go ahead and try it. I'm sure you'll figure out how to use the damage engine properly.

Just ask if there is any part that you don't understand.