Hm, if he wanted to debunk Christianity, and he's in modern-day London, he could easily say that the recent translations of Sumerian and Babylonian mythology show that the flood myth in Genesis was taken from the Sumerians by the Jews. Since we obviously don't believe that the Babylonian myths are literally true, and scientifically we know there was no world-destroying flood that covered the whole earth, then the bible is wrong. QED.
I think that's a bad faith argument but it's one I've encountered a few times.
Another common pro-atheism argument I see is that Old Testament God kills a whole bunch of people and orders genocides. If you take that statement, apply no nuance to it, and assume that any good deity's actions should be governed by the same ethics followed by modern humans, then that proves that He is terrible and should not be followed.
A third one is the argument from evil, which has been a bee in the bonnet of anyone arguing for an omnibenevolent and omnipotent deity. It goes like this: If God is all good and all powerful, then why is there pain, suffering and evil in the world? Those things are bad, therefore God cannot be all good and all powerful, therefore He is either not God or does not exist.
Nearly all the atheists I have met have held a combination of the above.
I think for the Adam and Eve + science debate at the beginning, at some point you had a first sentient primate, male and female, and they'd be the ones who did the deed, however primitive they were. I don't myself hold this position but that seems like the logical response.