Enemy AI problem

i have a problem with enemy AI, basically i've set the Navmesh system so they can avoid obstacles and when you enter a certain distance they stop moving and they shoot, but the problem is here, if they shoot and i'm behind wall, they don't run around the wall, how can i set a method where they avoid a wall if i'm not in their view when they stop at shooting? the idea would be a continous raycast, but i've noticed a continous raycast slows down performances a lot.

here's the AI code:

 private var personaggio:Transform;
 var agent: NavMeshAgent;
 var anim: Animator;
 private var velocita:int;
 var distanzaMinima:int;
 var distanzaMassima: int;
 private var rigid:Rigidbody;
 private var velocita_salvata:int;
 private var avvistato: boolean=false;
 var camera_nemico: Camera;
 
 function Start () {
 InvokeRepeating("attivaRaycast",0,1);
 personaggio=GameObject.FindGameObjectWithTag("Werner").transform;
 velocita=Random.Range(10,15);
 rigid=GetComponent.<Rigidbody>();
 agent=GetComponent.<NavMeshAgent>();
 anim=GetComponentInChildren.<Animator>();
 }
 
 function Update () {
 if(avvistato && personaggio!=null){
 CancelInvoke("attivaRaycast");
 anim.SetFloat("muovi",2);
 agent.Resume();
 agent.SetDestination(personaggio.position); 
   if(Vector3.Distance(transform.position,personaggio.position) <= distanzaMinima){
                 anim.SetFloat("muovi",0);
                  anim.SetTrigger("sparo");
                  agent.Stop();
                  
     } 
   }
 }
 
 function attivaRaycast(){
 
 var raycast_visivo:Ray[]=new Ray[Random.Range(20,20)];
 var hit_visivo:RaycastHit;
 for(var rv:int=0; rv<raycast_visivo.Length; rv++ ){
         if(rv==0){ 
             raycast_visivo[rv]=camera_nemico.ScreenPointToRay(Vector3(Screen.width+(rv*100), Screen.height*0.5, 0));
         }else{
             raycast_visivo[rv]=camera_nemico.ScreenPointToRay(Vector3(Screen.width-(rv*100), Screen.height*0.5, 0));
         }
         Debug.DrawRay(transform.position,raycast_visivo[rv].direction*100,Color.blue,0.5);
         if(Physics.Raycast(raycast_visivo[rv],hit_visivo,50) && hit_visivo.transform.tag=="Werner"){
         avvistato=true;
         }
         }
         }
         function Stop(){
         yield WaitForSeconds(2);
         }