Question about Devastate

[lebatt]

Ok I just received the skill yesterday.

I had a chance to try it once and then had to go to bed.
I had 2 mobs around me and i tried to use it, it said select mob. so i selected one of them and used it.
Only one was stunned and the other remained un hit. How come?
is there a special way to use it?

Please help elaborate on this.

Also did you invest any points into this skill?

Cooldown? buff time? dmg?