Enemy Boss movement AI

I am trying to get an enemy boss ship to appear on screen and attack my ship and also dodge my attacks sometimes. So far I have gotten him to attack my ship but hes stays on the edge of the screen.

Here is my code:

#pragma strict

// here are public variables for the enemy ship that can be accessed in the inspector
var health:int = 2;
var explosion:GameObject;
var expOrb:GameObject;
var enemyBullet:GameObject;
var expDrop:int = 3;
var hitSound:AudioClip;
var fireRate:float = 2.0;

//heres the private variable counter to keep track of time for fire rate.
private var counter:float = 0.0;

function Update () {
   //here we make counter count based on time for the fire rate
   counter += Time.deltaTime;

   //if the ship goes too far left, we destroy it.
   if(transform.position.x < -12){
      Destroy(gameObject);
   }

   //here we shoot 4 bullets if the counter counts higher than the fire rate.
   if(counter > fireRate){
      var custom1 = Instantiate(enemyBullet, transform.position - Vector3(0.5,0.1,0), Quaternion.Euler(-90,0,0));
      var custom2 = Instantiate(enemyBullet, transform.position - Vector3(0.5,0.1,0), Quaternion.Euler(-90,0,0));
      var custom3 = Instantiate(enemyBullet, transform.position- Vector3(0.5,0.1,0), Quaternion.Euler(-90,0,0));
      var custom4 = Instantiate(enemyBullet, transform.position- Vector3(0.5,0.1,0), Quaternion.Euler(-90,0,0));
      //to make the bullets spread, we add extra z velocity to each one to they all move on their own path.
      custom1.rigidbody.velocity.z = 3;
      custom2.rigidbody.velocity.z = 1;
      custom3.rigidbody.velocity.z = -1;
      custom4.rigidbody.velocity.z = -3;
      counter = 0.0;
   }

   //end of function update
}

//if a bullet hits the ship, the bullets sends us the hit message to trigger this function to bring down the ships health
function hit () {
   health -= 1;
   if(health != 0){
      if(audio.enabled == true){
         audio.PlayOneShot(hitSound);
      }
   }
   if(health <= 0){
      onDeath();
   }
}

//if health is 0, then this function is triggered to spawn some orbs, spawn the explosion animation object, and destroy itself
function onDeath () {
   Instantiate(expOrb,transform.position,Quaternion.Euler(-90,0,0));
   expDrop -= 1;
   if(expDrop <= 0){


       Instantiate(explosion,transform.position,Quaternion.Euler(-90,Random.Range(-180,180),0));
       Destroy(gameObject);
    }
    if(expDrop > 0){
       onDeath();
    }
 }

How do I add the movement aspect to it?

Solution

There are many ways to move an object, you can try:

Transform.translate (http://docs.unity3d.com/ScriptReference/Transform.Translate.html)
Modifying transform.position (http://answers.unity3d.com/questions/188998/transformposition.html)
Adding force to it (http://docs.unity3d.com/ScriptReference/Rigidbody.AddForce.html)
And many other ways that I may have not known yet.

For a boss in this kind of game, the movement you may want is either random movement or following the player. Both are not hard to achieve.

Random movement: just do a Random.Range(-1, 1) * bossSpeed * time.deltaTime and apply it to the boss' x position.
Following the player: get the player's x position then make the boss adjust to the position.

Then how to make the boss dodge the player's bullet sometimes? You can make the boss detect if a player's bullet is incoming by adding a collider thats stands in front of it. If a player's bullet collide with it, then random another number (eg. from 0 to 1). Afterwards, you just need to move the boss away if the random number is 1 and don't move the boss when the random number is 0.