Trump would need a significant number of independents plus all of the GOP base which would rather vomit than vote for him in some cases. The hard right isn't going to show up for him. Christians are not going to show up for him. And it's entirely likely that the establishment Republicans are going to sit this one out.
Advantage Hillary.
It depends.
Conservatives said that conservatives would stay home if they nominated Romney, and more conservatives voted for Romney than any other Presidential candidate in history. So I'm not sure if conservatives would stay home for Trump. In fact, given the passion conservatives here show for him, they may be more likely to go to the polls with Trump on the ballot.
The biggest threat Trump poses to Clinton is him peeling off blue collar Democrats, who - along with minorities - are Clinton's base. He may also bring out new voters, voters who generally don't vote. The other group to watch are young people. Young people would probably still vote for the Democrats, but Clinton doesn't seem to inspire young people in the way Obama did. Clinton isn't cool. Trump is "cooler" than Hillary. Clinton probably doesn't pull the same amount of minorities as Obama did. Clinton also has to be wary of the Left pulling her to far away from the center. The Democrat Party is shifting left, which threatens their viability. The final thing that could benefit Trump would be if terrorist activity increases.
OTOH, moderates decide elections. What appeals to the Republican base probably turns moderates off. Trump does relatively poorly amongst educated, higher-income Republicans, let alone moderates of the same demographics. Trump needs to win A LOT of white votes. Assuming minorities vote the same as they have the past few elections, Trump would have to win the same percentage of whites as Reagan did in 1984, which seems unlikely. Also unlikely is that he would do appreciably better with minorities than either McCain or Romney. The Democrats were also way, way ahead of Republicans in terms of technology in 2012. Have the Republicans closed that gap? Would it under Trump, a 69 year-old man whose primary advantage is amongst older, rural and less-educated voters? Another question is funding. Trump isn't spending any of his own money. Where will it come from? He probably would do better amongst small donors, but my guess is that big donors will sit this out. Of course, Trump doesn't come across as Presidential, and that matters also.
Don't discount Rubio. I think he will be the nominee, and I think Hillary would beat him, but I think it would be a lot closer than many Democrats think. People have dismissed him throughout his career, and all he has done is win. He's a better campaigner than Clinton, he'll do well amongst Hispanics, he appeals to moderates, he's young and attractive, and Americans look forward, not back. Clinton represents the past, Rubio the future.