Trump sued Maher over a joke challenge. Trump fulfilled the challenge, but Maher didn't follow through and make the charitable contribution that he promised. I would expect a judge to throw that out.
However, Trump has a case for libel, because he can prove a pattern of malicious behavior, rather than just an isolated incident. And, he can prove injury.
As I wrote above, I don't expect Trump to prevail. But, he can tie up CNN in discovery, and while the litigation is pending, they will have to be on their best behavior.
this is a pattern not a one time event....malice easier to prove when done repeatedly IMO.