Warren Buffett is no fan of Donald Trump, and this week, he made his stance known.

The Berkshire Hathaway (BRK.A) - Get Report (BRK.B) - Get Report head skewered the Republican presidential nominee at a Hillary Clinton rally in Omaha, Nebraska Monday, challenging his fellow billionaire to release the tax returns. Trump has insisted he must keep his returns under wraps because he is being audited.

"Now I've got news for him, I'm under audit too. And I would be delighted to meet him any place, any time between now and the election. I'll bring my tax return, he can bring his tax return. Nobody's going to arrest us, there are no rules against showing your tax returns. And just let people ask us questions about the times that are on there," Buffett said. "You're only afraid if you've got something to be afraid about."

He added that he doesn't believe Trump's reluctance to release his tax returns stems from the IRS. "He's afraid because of you," he told the audience.

Buffett also challenged Trump's campaign slogan -- "Make America Great Again" -- and the implication that the United States is somehow flawed or lacking. He quoted Trump: "No one knows the system better than me, which is why I alone can fix it."

"Well la dee da," Buffett said. "There's 325 million Americans, and if this guy leaves for Canada, it's supposed to be hopeless for the rest of us. He alone can fix it. It takes some kind of nerve -- or something else -- to really have the notion that out of 325 million people, you're the only one who can fix it."

Buffett looked to the markets and cited Trump Hotels & Casino Resorts, which was publicly-traded from 1995 to 2004, as en example of Trump's shortcomings.

"If a monkey had thrown a dart at the stock page, the monkey on average would have made 150%, but the people that believed in him, that listened to his siren song, came away losing well over 90 cents on the dollar, they got back less than a dime," he said.

The Berkshire chief executive also gave nod to Trump's ongoing attack on Khzir Khan. The Muslim father of a fallen soldier in Iraq spoke at the Democratic National Convention last week, telling Trump he had sacrificed nothing. Trump claimed over the weekend that he has made sacrifices.

"Donald Trump and I haven't sacrificed anything," Buffett said. "No member of the Buffett family has gone to Iraq or Afghanistan. No member of the Trump family has gone to Iraq or Afghanistan."

Buffett has been an outspoken Clinton supporter for quite some time. He joined her on the campaign trail in December, and before she had even officially announced her candidacy in 2014, he predicted she would wind up in the White House.

"I will bet money on it, and I don't do that easily," he said.