It was incredibly difficult to fulfil Auror training. If accepted into Auror training, prospective Aurors were required to undergo a stringent series of character and aptitude tests. These tests were done in order to show how they react well under pressure. Applicants were then required to train extensively in advanced magical combat and other elements of practical defence, and most likely methods of criminal investigation. Given that Harry, Ron and Neville became Aurors immediately after the Second Wizarding War, it is likely that they improved their capabilities in advanced defensive magic during the work hands-on.