"Masks work."

The way to win an argument with Leftists who say that isn't to counter them with contrary studies. It is to say: If "masks work," then feel free to wear one. But you have no right to force anyone else to wear one, ever. Especially if – as you insist – "masks work." After all, if they do – as you say – then they "work" for you and everyone else is none of your business.

That is how you counter a "masker."

Or – more to the point – it is how you shut one up. By telling him to go to bugger off, essentially.  After you have told him to bugger off, there is no longer anything left for him to do but to try to force you to. And you have forced him to admit that it's not about "masks working."

It is about the forcing.