You can use the look_at() function:
look_at ( Vector3 target, Vector3 up )
For example, you could add the following code to your mob script:
func _physics_process(delta):
look_at(player_reference.global_transform.origin, Vector3.UP)
Of course, this is not a perfect solution since it would also make the mob rotate up and down towards the player, so you might want to set a custom y value in the target Vector.