I don't know exactly how this thing is going to go, but my hunch is that Trump will try to make a show of strength tonight, hang back in the second debate, and come back strong again in the final debate.
Some are saying he's going to go easy and grow stronger over time -- that's not how he did it in the primaries. He came out swinging in the first debate, and in his announcement speech. After making the first impression, he hung back a little bit -- skipping the debate before the first primary, in Iowa. Then he turned it up to 11 toward the end of debate season after Lyin' Ted and Little Marco went nuclear on him.
There's too much excitement and hype over tonight, so he can't fail to deliver. That doesn't mean he's going to pummel her into submission, but it could mean key moments of focused, righteous anger like he showed about "New York values" and the Bush legacy ahead of the South Carolina primary.
Once the initial shock of the audience has worn off after this debate, they won't be so invested in the second one. So play that one more low-key. And the second debate will take
place during a relative slump in Trump's polling (whether he's ahead of
her or not). The second will serve more as an intermission for the audience.
Then end with a bang in the third debate. His polls will be gaining again by then, and improving over the second debate will leave a little "upward trend" in the minds of voters, giving them enough momentum to get them out on Election Day.