Help! Auto Spells

OK i'm in the last stages of my MAP, and I JUST NEED HELP WITH THE TOWERS THAT USE SPELLS...OK FIRST....I LIKE TO KNOW...how some MAPS LIKE POKEMAUL have AUTO Spells. there's like a unit, whenever it attacks, it purges the unit. I like to have auto-cripple, auto-banish....i think this **** involves triggers. If it does or not...TEACH ME

Event:

Unit is attacked.

Condition:

Unit type of attacking unit = Purger

Action:

Order PURGER UNIT to cast purge on attacked unit.

Make purge cost 0 mana, 0 cooldown, 0 casting time.

Ahh thx....:D Ice Climbers TD is Officially a sucessful map, still in beta though

Or make this:
Create a dummy "slow" autocast spell and make a trigger which casts a certain spell on the units then

still won't work...all that does is give a chance to cast the ability. Won't work unless it is a dark arrows/critical strike/bash etc. type spell.