To prove upon reflection at an optically denser medium the phase of em wave shifts by 180° if incident light is oblique to the surface?

This method is good but can't we use reflection coefficient method for oblique incidence as we did for normal incindence of light?

This is a general property of waves. If you have waves reflecting off a clamped point (like waves running on a string that you pinch hard at one point), the waves get phase inverte...