Just so that we are clear, I feel this entire discussion is a mistake. It began with Jormengand making a claim that Level 20 fighter could not defeat a Pit fiend and was liberally interpreted as an optimised fighter against a stock pit fiend played as a mannequin. What I am sure of, is that the OP really meant a Pit Fiend played like a Pit fiend. If you look at my post history in this thread this is precisely what I have adhered to. With that in mind, this entire exercise to my eyes is a strawman.
The MM makes it pretty clear how a Pit Fiend usually fight, so no, there isnt any mannequins here.
But is this then agreement on that a optimised fighter can indeed defeat a pit fiend?