The very toughest thing for me was to determine what is wrong and not rush into changes. Sometimes it is intention, sometimes it is the web page itself that is poorly optimised and sometimes it is just lack of authority on the website in general. It tooks more time than learning SEO techniques itself.Everyone learns SEO differently, and some concepts take more time to fully understand What was one SEO skill or concept that took you the longest to master? I think practical testing and experience often teach things that guides cannot