There is no doubt that the debate will be Trump vs both Biden and CNN. I'm sure Tapper will run interference whenever and however he can. He may give long winded questions in order to take time of the clock to shorten the debate time up for Biden and, of course, he will debate Trump himself in order to take some of the pressure off Biden. You can bet Tapper won't be debating Biden though, only Trump.
If I were Trump, I would come armed with several things straight off CNN which make Biden look bad and Trump good, including several polls, black support for one, and clips of Biden wandering around aimlessly. That will be hard for Tapper to counter. Trump needs to make CNN look like the biased rag that they are.