EnemySpawnManager.cs

    public List<GameObject> enemyFormations = new List<GameObject>(0);
	
	void Update ()
	{
        // If we have no enemies on the screen, get a random prefab from our list and spawn it then ensure it is active
	    if (GameObject.FindGameObjectsWithTag("Enemy").Length <= 0)
	    {
	        GameObject prefab = enemyFormations[Random.Range(0, enemyFormations.Count)];
	        GameObject formation = Instantiate(prefab);
            formation.SetActive(true);
	    }
	}