Yes, the global stock market will meltdown if Trump wins because he has ruined so many of his own businesses that he will destroy anything in the financial region as president. He is not a very smart businessman and an even dumber politician. No country will want to work with the US if he is elected.
Given the choices of the two realistic candidates for president, the bankers vastly prefer the policies of Hillary Clinton to Donald Trump. Clinton will continue the status quo, which is very very beneficial to bankers. Trump is a wildcard, and the financial markets hate wildcards. In the end I believe that Trump would also have policies that are mostly friendly to the banks, but the financial sector throws a fit every time something doesn't go their way.
The panic we are seeing in the global market following some polls showing Trump ahead is an indication of what could happen if he wins the election. However, there shouldn't be a global meltdown of the market should he in fact win but investors will be cautious and we could see scary short-term affects.
People promise panic every time there is a presidential election. People promise that the markets will crash, that we will go to war, and that the world will never be the same. It never ends up happening. Markets rise and fall. Seasons change. The world will keep turning no matter who the president is.