It can be intimidating to talk to someone that's smart that spouts nonsense about Bitcoin. Arguing with them is going to be difficult because they likely have flawed economic thinking that is at the core of their beliefs.
If you're arguing with a socialist, for example, they're likely to have some form of class struggle or oppressor/oppressed paradigms that guide all their thinking. If you're arguing with a mainstream economist, they're likely to view centralized monetary control as a good thing. If you're arguing with a gold bug, they're likely to deny the value of digital things.
It's a pretty heavy lift to argue with these people, because, most likely, they haven't studied Bitcoin in any depth. But if you want engagement, start with their assumptions about the current system. Do they know how it works? Do they understand that all money comes from loans? Do they get that inflation destroys savings and that it's a stealth taxation? At the very least, this should get them on a field that's a little more neutral and fact based. But it's also possible that they may understand all these things and still be against Bitcoin. What then?
You can move from there to whether freedom, particularly property rights are a good thing. Even the most ardent socialists don't like having property taken away from them. And really, this is a moral issue that's very intuitive. Usually, you can make headway that property rights, especially over savings is a good thing.
Finally, you can move to Bitcoin. Do they understand that it's decentralized? That it's digital and that digital things can have value? This is where you need to avoid the stupid economics of the mainstream, who like talking about velocity and unemployment and so on. Those are all irrelevant to the fact that Bitcoin is good money.