To want to have a normal relationship with another person is commendable. Unfortunately, they can't. Their inability to receive feedback is a part of their diagnosis. Attempts to do so will also cause them to escalate, as they are responding to the denial of their perception of reality, not the actual facts. How you communicate the facts won't matter to them, as the facts are disproving their view of reality. Merely having the facts presented to them is enough to trigger their escalation.
Most neurotypical persons have been conditioned by society to ask themselves if they may be in the wrong when someone responds in the ways someone with a narcissism diagnosis usually responds. Unfortunately, when interacting with a narcissist that isn't the case. Narcissists often blame others as a part of the narcissist's own maladjusted behavior, not because the other person is actually at fault. This reaction is actually a part of the diagnostic criteria used to diagnose the condition. To get around that quality would go a long way in helping clinicians treat the condition.
It is worth noting that narcissism, as a clinical condition, is so severe as to qualify as a personality disorder. The person's behavior will not be consistent with either your expectations or reality, but since narcissists go to elaborate lengths to conceal their own misconduct there is often no way to expect that at first. Since they stress their need to conform and to project success they often appear to be very well-adjusted. Many narcissists use their skills at deceit to advance professionally, and often become problematic for a formal environment to deal with. If you're encountering a narcissist in a professional environment be cautioned that they're going to be highly toxic to your career, as they will be willing to pursue you in a very vindictive fashion.
Unfortunately their inability to receive feedback is similar to the way a schizophrenic reacts when you deny their delusions. The narcissist's mind perceives themselves in an unrealistic fashion and is willing to tamper with records and evidence in an attempt to deny reality. If you contradict their perceptions of themselves and the context surrounding themselves the narcissist will escalate, as you are now evidence that their perceptions are wrong. They are responding to the denial of their perception of reality, not the actual facts. No statement that includes them in any way is going to be tolerated.
