I still don't have enough info, was your grade lower than everybody's else's grade or the average grade? Most schools have P, HP, and Honors. Look, it sounds like this site director did some pre-emptive staging, i.e. telling some students that they will get a "proficient" ahead of time, so when it comes time to get your grade you won't complain, but that doesn't make it right. We had a professor who said that he would "make our hearts heavy" or abuse us in different bizarre emotional days, how did he get to do this so long? Because he said he would, same case with you, if you complain to the dean then they will say it is his right to give grades based on what he thinks regardless of what others think because he states this ahead of time. It is ALWAYS a big red flag to have an attending to tell students how they go outside of the norm when it comes to being an attending, it means that they have a wood block on their shoulder and are getting complaints. Even if you are right, you might not "prevail" whatever that means for you, be a grade change or an apology or making it better for future students. Clership directors and site director often develop a feeling of total accuraccy of their students evaluations, which couldn't be further from the truth in alot of cases. Remember, a site or clerkship director states they want to control students grades, which is why they applied for the job, the power, and other attendings let 'em do what they want, they really don't care about the students.
You really need to think more about why your dean wants you to go talk to this person. They are thinking, "I need to get my nails done this afternoon, I am sooo tired of this little pretentious student, I will send this student back to talk to the site director who will make this student understand that we have the power to assign grades, not them. Was this student treated unfairly? I don't know, and don't care to find out, so take it out of my office." You can talk to more deans and professors, it may help future students, but in the end there is not much you can do. Third year is made to make you feel humiliated, harassed and powerless. Being a med student is different from being a college student, for some reason attendings will treat you worse than a dog, march you around like a toddler, and believe that med students should be seen not heard. If you complain they spin it and say you only care about your "career" when really you might want to fix a broken system which produces cold-hearted doctors. You will see what I mean in half a year when you are burnt-out addressing this issue and when it has taken a real emotional toll. You could be a worse situation, like really being harassed and then failed on a rotation, thank your stars that it did't go there.