Quaternion.Inverse

Description

Returns the Inverse of rotation.

    // Sets this transform to have the opposite rotation of the target

    var target : Transform;
    function Update () {
        transform.rotation = Quaternion.Inverse(target.rotation);
    }

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public Transform target;
    void Update() {
        transform.rotation = Quaternion.Inverse(target.rotation);
    }
}