One short sentence announcing what is happening, in general, absent of this particular feud. Two sentences stating your opponent's opinion, in a manner that is fair and they they would recognize. A few words stating that they are wrong (or very wrong, or catastrophically wrong, the case being.)
A promise of a detailed explanation later, but first, come context.
One or two or five paragraphs of necessary historical and philosophical context, as most questions deserving of a detailed response are really deserving of essay-length reflections. But this is the internet, five paragraph will have to do.
Two or three paragraphs articulating your rebuke to your opponent, in light of the historical context. The key here is to help the reader put two and two together -- resist the temptation to lean on any sense of authority. Now is not the time to be the delivery person of the Judgement Inc. Corporation. Present the argument fairly and sternly.
One line calling your opponent a ditwit or some such. By the time you get here, you've earned it