Or just do enough to get under his skin and trigger ranty old man mode in Trump. Or luck out and have it spontaneously trigger (which seems to be happening increasingly often these days).
I was really skeptical of Trump being able to keep a lid on ranty old man mode recently. I still am skeptical, but it is possible he could do it for the duration of the debate. Whether or not he will is the real question of the debate, barring something totally unforeseen like a complete meltdown by Harris.
In other words, the two most likely outcomes of the debate are:
Damage Trump, or
No big changes.
All other possibilities (help Trump, help Harris, damage Harris) are distinctly less likely.