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)