How To Get Object To Aim at Mouse in Top Down 3D Shooter

Here is an implementation of @MatthewA 's suggestion:

 public class MouseFollowTopDown : MonoBehaviour {
     
     void Update() {
         if (Input.GetMouseButton (0))
             RotateToMouse ();
     }
     
     void RotateToMouse() {
         Vector3 v3T = Input.mousePosition;
         v3T.z = Mathf.Abs(Camera.main.transform.position.y - transform.position.y);
         v3T = Camera.main.ScreenToWorldPoint(v3T);
         v3T -= transform.position;
         v3T = v3T * 10000.0f + transform.position;
         transform.LookAt(v3T);
     }
 }

Use this formula to calculate the rotation angle, target would be the mouse position in your case:

 float angle = (Mathf.Atan2 (target.y, target.x) * Mathf.Rad2Deg) -90f;

you might need to tweak it a little bit depending on your setup.

Goodluck

If this is a 2d shooter type thing, I think the easiest way is perhaps to use Camera.ScreenToWorldPoint, with Input.mousePosition as x and y, and the camera height for the Vector3 argument.

Then you could use transform.LookAt (or maybe RotateAround, with a bit more work to find the angle) to look at that point.

I know it's an old post, but this is still a valid problem.

I made a 3 minute tutorial about this with explanation and an example project.

My Solution

 using UnityEngine;
 
 namespace BarthaSzabolcs.IsometricAiming
 {
     public class IsometricAiming : MonoBehaviour
     {
         #region Datamembers
 
         #region Editor Settings
 
         [SerializeField] private LayerMask groundMask;
 
         #endregion
         #region Private Fields
 
         private Camera mainCamera;
 
         #endregion
 
         #endregion
 
 
         #region Methods
 
         #region Unity Callbacks
 
         private void Start()
         {
             // Cache the camera, Camera.main is an expensive operation.
             mainCamera = Camera.main;
         }
 
         private void Update()
         {
             Aim();
         }
 
         #endregion
 
         private void Aim()
         {
             var (success, position) = GetMousePosition();
             if (success)
             {
                 // Calculate the direction
                 var direction = position - transform.position;
 
                 // You might want to delete this line.
                 // Ignore the height difference.
                 direction.y = 0;
 
                 // Make the transform look in the direction.
                 transform.forward = direction;
             }
         }
 
         private (bool success, Vector3 position) GetMousePosition()
         {
             var ray = mainCamera.ScreenPointToRay(Input.mousePosition);
 
             if (Physics.Raycast(ray, out var hitInfo, Mathf.Infinity, groundMask))
             {
                 // The Raycast hit something, return with the position.
                 return (success: true, position: hitInfo.point);
             }
             else
             {
                 // The Raycast did not hit anything.
                 return (success: false, position: Vector3.zero);
             }
         }
 
         #endregion
     }
 }