On Thursday night, Donald Trump had his best debate of the 2016 campaign. He looked calm, sensible, and presidential. His Republican rivals seemed to have given up. “Maybe it shows I don’t have to be so, let’s say, flamboyant,” Trump mused in a Friday morning interview on Fox News. The key, he explained, was that “I wasn’t really hit. … When you get hit, you have to fight back.”

This was a perfect illustration of how not to debate Trump. If he wraps up his party’s nomination, only one person will stand between him and the Oval Office. That person—in all likelihood, Hillary Clinton—can’t afford to repeat the mistakes Trump’s Republican opponents made on Thursday. When she takes the debate stage against Trump, she has to make clear how dangerous he is. Here’s how to do it.

1. Understand what Trump is, and expose it. Every candidate has a character flaw. The flaw isn’t the whole person, but it captures what’s wrong with him. Marco Rubio, for example, is a coward. Ted Cruz is a liar.

Trump is a child. He’s narcissistic, insecure, and petulant. Those are three very bad traits to combine in a president. If you let Trump skate through a two-hour debate without revealing these traits, you’ve failed.

To draw out Trump’s inner child, you have to poke it. You have to injure his pride. When he feels wronged or threatened, he can’t control himself. He hits back, and he gets personal. He hurls insults: loser, choke artist, Lyin’ Ted, Little Marco. He tells his rivals they should get off the stage because they’re low in the polls. His outbursts are unbelievably puerile. In one debate, he said to Ted Cruz: “I know you’re embarrassed. I know you’re embarrassed. … I’m relaxed. You’re the basket case.”

This behavior puts off lots of viewers, including independents and Republicans. And Trump doesn’t just lash out at his opponents. He heaps scorn on candidates who have already left the race. He attacks moderators such as Megyn Kelly and Hugh Hewitt (“very few people listen to your radio show”). He even picks fights with the audience. Just get him started, watch him throw a few punches, and see how many people he can antagonize.

2. Bring out his insecurity. Trump is obsessed with his image. Long before he told us about the size of his Donald, he was bragging in every debate that his poll numbers were bigger than anyone else’s. He refuses to admit he lost Iowa. In every speech, he crows about how much money he’s made. To certify his intellect, he reminds audiences that he went to the Wharton School of business.

This self-preoccupation alienates many people. But to a debate opponent like Hillary Clinton, it would also be useful in another way. Remember how frustrating it was to debate Barack Obama in 2008? His ego was impervious. You couldn’t rattle him. Trump is Obama’s opposite: He can’t stand it when anyone thinks ill of him. So when you raise a topic that’s bad for him, such as Trump University, he latches onto it. Trump has to get not just the last word, but the last 200 words. He goes on for minutes, explaining sordid details viewers hadn’t even heard before. And the best he comes up with at the end is, “I’ve won most of the lawsuits.”

Take advantage of this defensiveness. Raise issues that Trump can’t resist addressing, though they can only do him harm. Here are two good ones:

(A) Mexico is going to pay for the wall. Nobody except for hardcore Trump supporters believes this, but Trump keeps insisting it’s true. It shows he’s either lying or delusional. Get him going on it.

(B) Atlantic City. Ask him about how he abandoned the city and stuck taxpayers with the tab. He always defends this episode by bragging about how cleverly he timed his exit and used bankruptcy laws to protect his company. His version of the story is bad enough. Let him tell it.

3. Put his belligerence in context. Trump is running as a strongman who will shake things up. In the context of Washington gridlock and corruption, that sounds good. Your job is to focus attention on areas where a wrecking-ball president would be a really bad idea.

This should be easy, because the first job of any president is national security. Trump’s anti-Muslim rhetoric (“Islam hates us”) and ideas (barring all foreign Muslims from entering the United States) would antagonize allies we need against ISIS. His threats to impose stiff tariffs on China and Japan would provoke trade wars. Blackmailing Mexico (“The wall just got 10 feet taller”) and changing America from the world’s policeman to the world’s thief (“Take the oil”) don’t sound terribly smart either.

Americans also don’t want a bullheaded president trampling civil liberties. If they don’t care about torture or targeting foreign civilians, remind them of Trump’s threats to “open up” libel laws and “close up” parts of the Internet. Point out his fondness for iron-fisted tyrants in Russia and China. When you combine these observations with the violence and loyalty pledges at some of Trump’s rallies, the thought of a resolute leader who imposes his will makes a lot of people queasy.

4. Communicate the breadth of his bigotry. A majority of the population isn’t black, Mexican, or Cuban American. Most Americans aren’t Muslim or Seventh-day Adventist. Half aren’t women. But when you line up Trump’s slurs and innuendoes against all of these groups, voters writ large begin to realize he’ll smear anybody (even if some dismiss this bigotry as Trump fighting political correctness). Trump isn’t a Klansman. He’s a salesman for whom prejudice is just another marketing tool.

5. Don’t get bogged down in policy disputes. Trump’s tax and budget numbers don’t add up. He’s a military ignoramus. He has no real plans for health care or education. Of course you want to talk about these things. Don’t. Trump’s Republican opponents pressed these issues against him in several debates, without effect. Debating policy with Trump just makes him look presidential. His fatal flaw is temperament, not substance. And there are so many damning things to talk about—his scams, his bankruptcies, his aspersions against Latinos, all the countries he has already offended—that every minute you spend on the mere impracticality of his so-called plans is a minute you could have devoted to something worse.

6. Be the adult. The hardest trick to pull off, and the most important, is to provoke Trump without looking petty yourself. You can’t bicker with him onstage, as several Republicans have done, or sink into a pleading tone, as Jeb Bush did. When Trump goes after you personally, you have to be what he isn’t: poised, calm, rational, mature. You get to stand opposite him as a direct point of contrast. You can do what no one else can do: expose the absurdity of a Trump presidency by modeling what a president should be.

In the Republican debates, one candidate got this right: Carly Fiorina. She rebuked Trump without matching him. She chose her words sparingly and kept her voice even. She let Trump’s insults and dominance displays alienate the women watching at home. She used his taunts, her understanding of foreign policy, and the force of her counterexample to eviscerate him.

But she wasn’t you, Madame Secretary. She didn’t have your experience, your money, or your political machine. And she didn’t get to face Trump one on one. You do. Go get him.

Read more Slate coverage of the 2016 presidential election.