If I were Trump I would tell the world every chance I got that Biden is brain dead and not capable of a Debate. Then when Biden doesn’t show up for a Debate, Trump can say “see I told you he was brain dead”. If Biden does show up, it will be obvious to the world. Oh, and by the way, Biden is brain dead.
And if I were Biden, assuming there are debates to begin with, I would give a short, concise answer to every question and yield the balance of my time to Trump. The more either man speaks the better the chance for a flub.