Calling script method for collider in collision callback

I want to access the opponent game object on collision callback, I am able access the root game object through collider.trasnform.root but how can call a script method on it ? I am getting this error on script compilation " Cannot convert type `UnityEngine.Transform' to 'Opponent' " where 'Opponent' class defined under "Opponent.cs".

Method :

void OnCollisionEnter(Collision collision)
{
Opponent opponent = (Opponent)collision.collider.transform.root.gameObject;
opponent.reduceHealth();
}