How Trump would do against Hillary depends on his campaign. He might need help on tactics and organization, but the GOP could help him there - if the party wanted to. In all other areas, he has the opportunity to reach the voters and change their opinions.
I hope this thread will stick to the issues and stay out of the Trump-Cruz wars. This was a good question and a fair, fact-based question. I would love to see good answers.
[Something similar could be asked about Cruz, but with slightly different issues.]
Let me offer what I think is a fair answer. Trump already has most likely 90-95% name recognition. Most of those folks have an opinion of him. For this reason the polls can be considered fairly legit. The results are very very unfavorable. It would take almost a miracle to change the perception of the electorate.