Quaternion.Inverse  

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

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public target as Transform

    def Update():
        transform.rotation = Quaternion.Inverse(target.rotation)