Disagree with the claim, not the person

The mechanical move is keeping the disagreement attached to the specific thing rather than to the person or their competence. "I do not think that follows, because of X" is about a claim. "I think you have got this wrong" is about them, and it will be answered as such.

Then say what would change your mind, and mean it. Naming the evidence that would move you does two things at once: it shows you are arguing rather than defending, and it gives the other person somewhere useful to go instead of a wall.