Complex sentence: Hallinger (2003) has argued that the transformational leadership, where leadership is shared with school staff and empowers them, is a good supplement to the instructional leadership approach that focuses solely on principals and top-down strategies.
Split into simpler sentences: Hallinger (2003) has argued that the transformational leadership approach, in which leadership is shared with school staff; this approach is said to empower staff. Transformational leadership is a good supplement to the instructional leadership approach that focuses solely on principals and top-down strategies.