How do I answer "why were you fired?" diplomatically?

I worked at an at-will law firm. A new employee was hired in my department and I was asked to show how the department operates. I was okay doing so until I realized all questions were due to lack of basic word processing skills, our job description was "legal word processor". We had numerous meetings with HR during which I was told that the staff trainer was busy and I should be patient. I refused to provide "job" training. It escalated and I was so stressed that I closed-up and stopped interacting, except when I absolutely had to. Soon thereafter I was terminated for causing a hostile environment. I worked at this job for 6 1/2 years and always got great performance reviews and merit raises, including my anniversary on January 2012. How do I phrase my answer on interviews without sounding critical or whiny?