Quad Object needs to look at character rotating one Axis

Hi everyone, new to the comunity, so here I go.

We have a problem on our game about Quad looking at the camera just rotating one axis of the object. As you can see in the picture you will understand:

http://puu.sh/gxh78/d7137252b4.jpg

What we need is to make the Quad look at the character screen, just rotating on the Axis X of the object, not the world. We tryed to figure how to do it but we don't have any other option for doing that.

The code we use right now is:

 var newRotation = Quaternion.LookRotation(transform.position - target.position, Vector3.forward);
     newRotation.z = 0.0;
     newRotation.y = 0.0;
     transform.rotation = Quaternion.Slerp(transform.rotation, newRotation, Time.deltaTime * 8);

We would like to appreciate any kind of help or recomendation about how to solve it, even if we have to change the script.

Thanks for your time and take care.