First, determine if an argument is worth having. If your interlocutor cannot imagine they would change their mind under any circumstances then the debate is not worth having. You can't reason somebody out of a position they didn't reason themselves into.
Ask: Under what circumstances would we change our minds? Ask: What definitions are we using for key terms in our discussion? Ask: What do you mean by that? Ask: Why do you think that? In other words: What has caused you to think that and what are your reasons for thinking that?
No productive conversation can take place if there is a disagreement on definitions of key terms, underlying assumptions are hidden, and an interlocutor is not open to updating prior hypotheses.