Summary: Avoid arguing with anti-Islamic person thinking that you have some duty to defend Islam. Rather call them to Islam repeatedly, or otherwise leave that place.
For example:
Atheist: Quran has contradiction xyz.
You: I trust in Allah the creator on Heaven and Earth and lord of everything. [avoid bring up a counter argument]
Atheist: Yeah, once I was a muslim too but then I realize the wrong blah blah.
You: Fear Allah, because every one of us will have to stand before Him. [you repeat your call]
Atheist: You are dumb.