How to block one collider behind another in Unity

Is there a way to block one collider object behind the other in a 2D Unity game?
I have colliders for enemies walking on screen but if the enemies walk behind barriers (which have their own colliders), the enemy colliders still respond to the player bullets. So basically i just want to expose the enemies only when they are not behind any barriers.
Currently my bullets go through the barriers and detect the enemies.
I have placed enemies and barriers on separate UI panels and layers but that is just distinguishing them visually. From collision point of view how do i make one (barrier) take priority over the other (enemy)?

You could use Raycast methods. You can cast a ray from the player position to the enemy position, if it detects a wall in the middle you don't allow them to shoot.

public LayerMask wallsLayer;

bool HasPathFreeOfWalls(Transform player, Transform enemy, float reticleRadius)
{
  Vector2 direction = enemy.position - player.position;
  Vector2 origin = player.position + (direction.normalized) * reticleRadius;
  float distance = direction.magnitude - reticleRadius;

  RaycastHit2D rayHit = Physics2D.Raycast( origin , direction, distance, wallsLayer);

  return rayHit.transform == null; // if does not hit anything, it means that the path is free of walls
}

Edit to match game description

One way of doing this is offsetting all the colliders to be above the pivot points of their respectivelly gameObjects, like this.

Then you cast a CircleCast around the reticle to detect all objects there and checks which one has the lowest Y position, meaning the one that is in the front. Like this:

public LayerMask enemysAndWallsLayers;

bool HasEnemyInFront(Vector2 reticlePosition, float reticleRadius)
{
    RaycastHit2D[] enemysAndWallsHitted; // holds the results of the circle cast
    ContactFilter2D collisionFilter = new ContactFilter2D(); // filter user by the circleCast
    collisionFilter.layerMask = enemysAndWallsLayers;
    collisionFilter.useLayerMask = true;

    Physics2D.CircleCast(reticlePosition, reticleRadius, Vector2.zero, collisionFilter, enemysAndWallsHitted, 0);

    GameObject closestGameObject = null;
    float closestY = Mathf.Infinity;

    foreach(RaycastHit2D objHitted in enemysAndWallsHitted)
    {
        if(objHitted.transform.position.y < closestY)
        {
            closestY = objHitted.transform.position.y;
            closestGameObject = objHitted.transform?.gameobject;
        }
    }

    return closestGameObject != null && IsGameObjectEnemy(closestGameObject );
}

The IsGameObjectEnemy() fuction have to be implemented by you in a way that you can tell if it is an enemy or a wall

I commented this, but I thought as it might solve your problem, I would offer it as an answer.

On the function that checks the collisions, place the barrier check before the enemy check. E.g:

if(collider.gameObejct.tag == "Barrier") {//Destroy object} 
else if(collider.gameObject.tag == "Enemy") {//Damage enemy}

This way it will check if it hits the barrier first. If it hits the barrier, no life will be taken from the enemy.

Hope this helps. Happy Coding!