AimAssist.cs

using UnityEngine;
using System.Collections;

public class AimAssist : MonoBehaviour {

    void Start () {
    }

    void Update () {
        Vector3 mouse_pos = Input.mousePosition;
        Vector3 player_pos = Camera.main.WorldToScreenPoint(this.transform.position);

        mouse_pos.x = mouse_pos.x - player_pos.x;
        mouse_pos.y = mouse_pos.y - player_pos.y;

        float angle = Mathf.Atan2 (mouse_pos.y, mouse_pos.x) * Mathf.Rad2Deg;
        this.transform.rotation = Quaternion.Euler (new Vector3(0, 0, angle));
    }
}