Purushottam Aggarwal sir is best faculty for ca final costing. his books covers all the concepts and question from icai study material. teaching technique is also exam oriented and sir provides many short tricks which will help you in exam. Also the best thing i liked about sir is points to remember which helps in revising in exam time.
Sankalp kanstiya is worst teacher for costing. his teaching is not upto mark for ca final . many concepts were not a t all covered in his classes and poor explanation of some topics. i also bought his classes but now i am doing with self study. his many agents are active in social media who will suggest he is best teacher but trust me beware of such fraud teachers.