Making "I" statements does not exclude setting boundaries, which can be "I" declarations. You don't respond to that defense, but state what you ("I") will do if abuse continues....and do it. My book on narcissism gives specific steps and scripts to deal with a highly defensive person using Strategic Transactional Communication. Dating, Loving, and Leaving a Narcissist: Essential Tools for Improving or Leaving Narcissistic and Abusive Relationships https://whatiscodependency.com/dating-loving-and-leaving-a-narcissist/