I can do it on the individual level, but probably not to any large group.
I usually start with saying something like “I’ve heard all ---- are alike, do you think that’s true?” (substituting some grouping this person is likely to insist is multifarious for -----) and then following up without giving them a chance to answer with “I don’t think it’s possible that all unions could possibly be alike, there have to be good and bad ones, because that’s how people are, right? If some unions will have corrupt featherbedding leadership they’ll get all the press, while the ones that do good things like support widows and orphans of workplace accidents will be ignored. Because that’s what the media is like, right?” And then let them rant a little, before talking about some of the stuff that collective bargaining has historically given us.
I try not to directly confront with polar opposite views, that’s bad strategy. Never put your opponent in death ground, as Sun Tzu might say (or more colloquially, a cornered rat fights hardest.) Instead give them a way out - “well, what you say is probably true about some unions, or maybe even most modern unions, but historically unions made our nation much better than it was, so unionization is a powerful tool we shouldn’t throw out - we should reclaim it for the better, only get rid of the bad union leaders so the good ones can prosper.”
And I try not to go too far. You don’t need to persuade the person to see things exactly as you do right now. If you plant a true seed, it will bear true fruit, in time. Be patient, plant the seed and back off.
It’s much harder if you respect the person too much to be willing to manipulate them psychologically, or if you are speaking in public (or on an online forum) where you have to very meticulously tell the truth or you’ll discredit your cause. In a bar argument you can be sloppier with language and more manipulative without doing anyone any harm.