issuing spell order problem.

Hello I'm trying to get Illidan to cast rain of fire and it works but, he starts casting it then it's interrupted because he starts attacking and does other things instead.

I need him to stop and channel the spell untill it's over, is there any way to do this?

The trigger looks like this btw

Is the unit owned by a computer player? If so then you might have to change it's owner to a neutral enemy to prevent the casting from interrupting.

Yeah it's neutral hostile, if I change it to neutral passive will the spell work on the attacking units though?