Basic "shoot to target" in Unity 3D

First, let's create a new Unity 3d Project, and select 3D.

Let's add a terrain GameObject-3D-Terrain. Import Characters package Assets-Import Package-Characters, select all, and click Import. Find the ThirdPersonController in the Project pane (use the search tool if you can't find it) and add it to the scene.

Create a Cube GameObject-3d Object-Cube. Name it ShootBox. Create a Sphere GameObject-3d Object-Sphere, set its scale to 0.5 for x, y and z. Name it BulletPlaceHolder. Create a folder Resources. Drag the two recently created objects into the Resources folder. Create a Scripts folder. Create three scripts: Bullet.cs, Player.cs, and ShooterBox.cs and add their respective code.

Bullet.cs

using UnityEngine;
using System.Collections;

public class Bullet : MonoBehaviour
{
    private float Speed = 3.5f;
    public GameObject Target = null;
    // Use this for initialization
    void Start()
    {

    }

    // Update is called once per frame
    void Update()
    {
        if (Target != null)
        {
            float step = Random.Range(1,5) * Time.deltaTime;
            this.transform.position = Vector3.MoveTowards(this.transform.position, 
            	Target.transform.position + new Vector3(0,0.5f,0), step);
        }
    }

    public void OnTriggerEnter(Collider other)
    {
        Debug.Log(other.name);
        Player playerComponent = other.GetComponent<Player>();
        if (playerComponent != null)
        {
            int newHealth = playerComponent.Health - 1;
            if (newHealth == 0)
            {
                Time.timeScale = 0;
            }
            else
                playerComponent.Health = newHealth;
            GameObject.Destroy(this.gameObject);
        }
    }
}

Player.cs

using UnityEngine;
using System.Collections;
using UnityEngine.UI;
 
public class Player : MonoBehaviour {
    public int Health = 100;
    public Text HealthText;
// Use this for initialization
void Start () {
}
 
// Update is called once per frame
void Update () { 
}
 
    void OnGUI()
    {
        HealthText.text = Health.ToString();
    }
}

ShooterBox.cs

using UnityEngine;
using System.Collections;
 
public class ShooterBox : MonoBehaviour {
 
// Use this for initialization
    private GameObject player;
    private float lastTimeShooted = 0;
    public int ShootIntervalInSeconds = 15;
void Start () {
        this.player = GameObject.FindGameObjectWithTag("Player");
}
 
// Update is called once per frame
void Update () {
        if  ((lastTimeShooted + ShootIntervalInSeconds) < Time.time)
        {
            GameObject loadedResource = Resources.Load<GameObject>("BulletPlaceHolder");
            GameObject newInstance = GameObject.Instantiate<GameObject>(loadedResource);
            newInstance.transform.position = this.transform.position;
            Bullet bullet = newInstance.GetComponent<Bullet>();
            bullet.Target = this.player;
            lastTimeShooted = Time.time;
        }
}
}

Add the Bullet.cs script to the BulletPlaceHolder prefab. Add the ShooterBox.cs script to the ShooterBox prefab. Add the Player.cs script to the ThirdPersonController in the scene. Create a Canvas object and add two objects to it: HealthLabel and HealthText.

Go to the ThirdPersonController in the scene, in the inspector find the Health Text field below the Player script and assign the recently created HealthText object.