Problem with collision detection

I am making a simple game where you shoot a fireball at a sphere monster that follows you. My code for the collision and the monster's following is this:

var thing : Transform;
var speed : float = 3.0;
function OnControllerColliderHit(hit : ControllerColliderHit)
    if(hit.gameObject.tag == "Fireball")
function Update () 
    transform.Translate(Vector3.forward * (speed * Time.deltaTime));

it doesn't destroy the monster and it doesn't say hit when i play the game and shoot at the monster. I made sure everything that needs it has colliders and everything else works (except for the fact that the monster goes through the terrain's hills)

Why are you using "OnControllerColliderHit"? From the documentation:

OnControllerColliderHit is called when the controller hits a collider while performing a Move.

You need to change that to either `OnColliderEnter(Collision hit)` or `OnTriggerEnter(Collider hit)` depending on how you have your game set up.