I honestly believe that if you are a Hilary Clinton supporter, the best thing you can possibly do is register republican and vote for Trump in the primaries.
Trump wont win the election. He probably wont win the republican nomination. If he does well enough, however, I'm confident he will run as an independent. He has no support outside of a small part of the republican party, but taking that vote away from the republican nominee is probably enough to push the election towards the democrats. That doesn't happen unless he does reasonably well and stays in the public eye. Even better if he winds the republican nomination but gets shut down by the republican party (which can happen, theoretically). Overall, your vote in the primaries holds more meaning if you vote for a republican candidate rather than a democratic one.
On the flip side, Hillary is pretty likely to win the democratic nomination. I'm still debating between Hillary and Bernie myself, but I'd rather see Bernie get elected. While his economic policies are more socialist that I like, he doesn't have complete control, so given the house and senate not being quite as socialist, it should even out so it gets to a reasonable place (a huge over-simplification, granted). That being said, the odds of him winning a general election is less than the odds of Hillary winning, because he mainly appeals to the left side of the democratic party, not to independents.