The bar has already been set so low, anything other than a total disaster will make Joe appear to be the winner.Biden does have substantial cognitive decline, but despite that it would not surprise me if he makes few mistakes tonight. What I'm saying is don't go in expecting he'll spend every minute fumbling as if mid-stroke.
I think Trump should do his best (He is incapable, but we can hope) of staying above board, very professional and presidential. It's possible to be come across as dominating while also being classy and in control. Don't let Biden get under his skin.
Also I would not be surprised AT ALL if Biden does decent tonight, he will skip the remaining debates and cite COVID safety. This way he limits his public exposure, but also cannot be accused of refusing to debate.