1. If the moderator starts in on his taxes, he should simply point out that the New York Times violated the law by releasing confidential tax information (whether this is true or not is irrelevant), so his lawyers advised him not to release them until an investigation into the matter is complete.
2. If the moderator starts in on the "racist" angle, he has more than enough ammunition to point out that he is working for all Americans.
3. If the subject of immigration comes up, he should really clean up by making the case that he wants to protect all Americans -- black, white, and Hispanic -- from an invasion of immigrants who can't be vetted properly, many of them terrorists. He needs to cite specific examples of Islamic terrorist immigrants like the Boston Marathon bombers, the immigrant from Afghanistan who planted those bombs in New York City a few weeks ago, the Somali terrorist in the Minnesota shopping mall, the Turk who shot those five people in the Washington shopping mall, etc.
What you described is a block. That’s good. But he needs to follow up each block with a deflect onto Clinton.
The racist angle - I’m working for all Americans, etc. Hillary Clinton has said that all Americans are racist. She called black men predators. She thinks half of my supporters are irredeemable deplorables. Bernie supporters are basement dwellers. Mrs Clinton thinks there’s something wrong with anyone who doesn’t support her.
You get the idea. Block and deflect. He has plenty of ammo.