It’s true that this is most often a tactic used to rattle your cage & confidence. One good way to handle it is to tell them straight off that if they depart from the topic they begin with the discussion will end and you will walk away. This makes it plain that you will not allow the “scattergun” stunt of throwing rapid-fire assertions out without waiting to finish the discussion of the first one.
In online forums, it’s easy to do because you simply point out that it is off-topic and that they are welcome to create a thread for it and you’ll discuss it but refuse to undertake it in the current thread. Fortunately, the Mods on most good forums will also intervene to reinforce that. If not, it’s not a very good forum. Be somewhere else.
In person, just breathe slowly and maintain a prayerful attitude like “Give me the right words and wisdom O Lord.” (Luke 21:15)
Understand that this is most often the person(s) idea of “witnessing” to you, to argue you down, not allow you to properly respond, offer offensive and inflammatory remarks, and unless you fall on your knees and “accept Jesus Christ as your personal savior”, they don’t really give a rip about Christian charity or common civility. The harsh truth is that it’s an ego-feed for the carnal Christian nad it makes them feel like hotshots to tell themselves you couldn’t answer. (I’ve had it happen to me several times.)
My favorite response is often to just remain silent until I have their attention and simply say. “I have answers for every one of these things, but you don’t seem to be interested in discussing anything so when you decide you are, come find me and I’ll deal with anything you have one topic at a time. Until then…goodbye.” And just calmly turn on your heel and walk away (ignoring anything said that you might overhear). Do not return to the discussion because you want to make it very plain that they have blown their “witness” with you, and lost their chance for now.
Then pray a Divine Mercy Chaplet for them and wait for your next chance to share your faith. One thing you have to remember is that we are under no compulsion to accept rude and uncharitable behavior, (any more than that excuses us to respond in kind…).
Pax tecum,