2D Sprite rotation but still face the camera?

Hello, i am currently have a 2D sprite which is an enemy indicator arrow (radar effect). it faces towards the camera in my 3D game and it\ moves around the viewport edges as the enemy outside of the viewport changes location in relation to the viewport.

Now the problem i am having is that i can't get that same indicator arrow to rotate to point at the viewport border (where the enemy is) while still keeping its face towards the camera. So how would i rotate a 2D sprite while still facing the camera?

Right now i am trying to make the sprite look towards the camera and then change the z rotation but this doesn't give the result i want.

 transform.LookAt(camera.transform.position);
 transform.rotation = Quaternion.Euler (0, 0, angle * Mathf.Rad2Deg);