auto-attack spell

I have been looking at the spells trying to find a good spell to base a auto-attack spell on of for a trigger spell.
I'm just unable to find one. Any ideas how I should do this?

Slow, Fairy Fire, etc.

Something autocast that targets enemies.

Try the "web" that crypt fiends got, and make it target ground and air, coz the fiends used to web its target as soon as it is within range, be4 its first shot.

im not sure about this, but if the enemy is channeleing a spell such as starfall and you web them, it might make them stop casting the spell, so web isn't a great idea

Actually the unit never has to actually cast the web in the first place. If you set the casting time of web to more than 0, and have your trigger fire when the unit begins casting web, you can use the trigger stop the web from being launched.

also, make sure that when the unit attacks with the web ability, make sure that you have the attack animation instead of the spell animation