Efforts to prevent Trump from becoming the nominee, or defeat him with a better candidate if he becomes the nominee, usually focus on a couple of scenarios. One would be to deny him the nomination by means of something like a rule change at the convention in July. Another is to run a third-party candidate against him, with the hope of preventing both Trump and Clinton from getting the 270 electoral college votes they'd need. That would place the decision in the hands of the House of Representatives.
Both scenarios are highly unlikely to occur. The Republican party has the ability to deny Trump the nomination in a variety of ways, but the people in a position to do it (e.g., the delegates going to the convention) don't seem to have the will to carry it out. And a third-party candidate would have a hard time beating both Trump and Clinton in enough places to prevent both from getting to 270.
A third option that I've seldom seen discussed is also unlikely to work, but seems more promising. Convince Trump to drop out. As John Fund mentions in an article I linked earlier today, there's a good chance that information on Trump's tax returns will be leaked if he doesn't release the returns. Try to persuade him to avoid that development by dropping out. Persuade him that his likely loss to Clinton in November would do a lot of damage to the Trump brand and his reputation in the business world. Explain that, if he remains in the race, a lot more effort is going to be made to look into his relationship with Jeffrey Epstein. And so on. It would be good if some individual or group could have discussions with Trump about dropping out. But not many people would have that sort of opportunity. What more people could do, though, is raise these issues publicly. Write editorials in newspapers. Publish articles on web sites. Publications could assign some of their staff members to researching topics like the ones I've mentioned above. Make it known that the issues will continue to be investigated as long as Trump remains in the race.
One advantage to this kind of approach is that it also makes the first scenario discussed above (i.e., the Republican party denies Trump the nomination) more likely. Simply repeating objections to Trump that have already been raised probably won't convince the delegates at the convention and others involved in the process to deny Trump the nomination. But if new information is turned up about Trump's tax returns, his relationship with Epstein, etc., that has more potential to change some minds.
Another advantage to the approach I'm suggesting is that it's still applicable if Trump gets the nomination in July. If he dropped out in August, for example, that sort of late drop-out would be better than his not dropping out at all.
Will the approach I'm suggesting work? Probably not, but I think it has more potential than the other options. I think all three approaches I've mentioned in this post should continue to be pursued. But most resources should be focused on the third option.