Hi.
Beautiful Soup and Nokogiri are actually overkill for this particular application.
You did not describe what kind of output you want. You did say XML but do you want individual XML files, for example?
If you will be entering the URLs yourself, I suggest a small Ruby + Rails application, basically a web page for your browser with a simple text box to type in your URL.
If the URLs will be gathered automatically, you are looking at more of a web scraping app which is a bit more complex.
If you just want to type in a URL and get one or more XML files for output, I can build you a custom parser in Ruby that will be much faster than either Beautiful Soup or Nokogiri are capable of. It should take me less than a day. You would need to install Ruby on your computer, which is easy.
Also, I will need to know what OS your are running.