[K]eep in mind that the presuppositionalist is not approaching you with an eye towards reasoning together to get to the truth. He's admitted as much by saying that he sees you as deranged and in need of some kind of personal confrontation. He is not entering the discussion in good faith, as a co-investigator. (Van Til has an essay on this that is quite enjoyable, featuring Mr. White, Mr. Black and Mr. Gray, which makes it quite plain how they see such attempts. It's one of the few things I've read by the presuppositionalists that I found enjoyable.) In an ordinary intellectual discussion among friends, it's fine to speculate on things, refining one's ideas as one goes along; but in this sort of encounter they will try to commit you to as much as they can, to force you off balance. So when I say "think in terms of tactics" I mean: keep focused on how your words will be used by the presuppositionalist. In effect, think of those famous Miranda warning words: "Anything you say can and will be used against you." Don't commit yourself to more than you need to in order to make your points.
[h/t to Arthur Vandelay]