In 1996, writer David Irving sued an American academic for libel, on the basis that she claimed (in effect) that he was a Holocaust denier. The lawsuit effectively became about whether or not very large numbers of Jews died at Auschwitz. Lipstadt’s publishers conducted the litigation on her behalf. Irving acted as his own lawyer.
Lipstadt’s publishers gathered a very large body of historical evidence together to make it plain that Irving was not only wrong, but that no reasonable historian could hold the views he had expressed (thus demonstrating he was, in effect, not a legitimate scholar but a denier).
Much of the evidence can be found at this website, where the judgment in the High Court (which vindicated Lipstadt) can also be found.
Of course, as observed by lissener, deniers are not going to be swayed by direct reference to such a site. They assimilate, distort and then mock any evidence that does not fit within the conspiratorial world view they hold. But there it is, and it may provide ammunition in the debate you are having.
The key problem had in debates with these people (and all CTers) is that they are masters of a morass of tiny details in the evidence (some of which are true and some not) which no-one else can sensibly expect to have the energy to spend their life mastering. They thus create the illusion of dominance in debate.
They also are fixated by a style of reasoning that has been described as “crazed positivism”; seizing on one tiny detail or a series of them and then proceeding on the premise that such matters can somehow turn the world upside down and demonstrate that black is white.
Of course in principle, it is possible that one piece of new evidence can overturn what previously had been thought to be an unassailable understanding of the nature of things. Science always leaves open the theoretical possibilty that even so well-established a principle as that of gravity might be better understood in the future.
But in reality, where the accepted understanding is very well established, the “tiny detail” upon which deniers place reliance always turns out to be false, or a misinterpretation of the evidence. The world just does not work according to the principles of reasoning applied by deniers and CTers, but they have well-developed techniques of rhetoric to artificially inflate the appearance of credibility of their arguments.
I wish you success, but I have grave doubts that you can penetrate the impenetrable world-view of these people.