In the United States, it does not matter how you save any evidence; the other side will essentially always be permitted to question its authenticity. Even if they don't question it, a judge or jury would still be free to do so.
That said, the standard means of saving this kind of evidence would be to make a screengrab or print it to PDF, and to attach that to an affidavit in which you swear that the image is an authentic representation of the content of the web page as of whatever date and time.
If you want something that is harder to question, you could also ask some independent third party to do so. There are, for instance, archiving services like archive.org and perma.cc that will copy a page and store it indefinitely, largely removing the question of whether you might have manipulated the page in any way.