Defamation lawsuits require different standards of proof depending on the status of the victim and the accused. The women accusing Donald Trump of sexually assaulting them could potentially bring a defamation lawsuit against him simply by virtue of the fact that he called them "liars" publicly. If a judge or jury finds that the women believe in good faith that Trump did assault and/or harass them, they are not "liars" and Trump's statement that they are is slander and a proper subject for a lawsuit against Trump. Trump could very well lose such a lawsuit should any of the women accusers choose to pursue litigation.
In contrast, should Trump choose to pursue litigation against any of these women, he would likely lose since he stands in the capacity of a public figure, or possibly a public official. U.S. Supreme Court precedent holds that a defamation suit will not stand against a citizen who makes a false statement about a public official or figure. Since public officials are often subject to criticism and harsh words, quelling all statements by the public would inhibit Free Speech and create an endless string of lawsuits. Instead, a public official must prove that the citizen had "actual malice" to deliberately disseminate false information about him to cause harm - a very hard burden to carry. Trump may very well bring lawsuits against these women, as he has threatened and as is his modus operandi when trying to bully people into doing what he wants, but in this case, he would likely, and luckily, lose.