How to make an ability become auto cast?

[plazma019]

I want my hurl boulder become ac.

Also I want my hurl boulder targets many enemy.
The important thing is to become auto cast

 

[Jampion]

You have several options (using a dummy autocast spell, casting it on attacking, ...) but it really depends on which behaviour of autocast you want (random nearby enemy, only when target attacks, ...).