If I were an atheist, the first thing I would pick on would be the Bible. Many Christians place so much stock in the Bible, sometimes even more than they do in God, that when an atheist can overcome a Christian's trust in the Bible, God often goes with it. I am actually writing a book on the proper way to understand the whole of the Bible which is immune to attacks from atheists. The book also has to overcome a lot Christian thinking that developed in the 1900’s about the Bible that is incorrect. Overall though, I want to point out that God’s existence is completely independent of how much of the Bible is accurate/true or not. Let’s imagine for a second that the Bible is completely false. What does that tell us about God’s existence as a category? Nothing! Now, it would undermine particular beliefs we have about God, but God would still exist.
Second, I notice a lot of atheists spend time on the 'Problem of Evil'. This problem has been around forever, and from what I can read, the Theists (those who believe in a personal God) have the upper hand on this discussion. The 'Problem of Evil' is usually stated in such a way to show that God cannot be all good, all powerful and allow such evil in the world. However, a great philosopher by the name of Alvin Plantinga has laid the argument to rest at the logical level, which is largely based on free-will. There is nothing contradictory about those things.
Third, anyone who tries to prove a negative is in for an uphill battle. That is why in my debate with Professor Larry Shapiro from UW Madison, he did not argue that God does not exist. He argued that there is no justification for thinking that God does exist. Thus, another way to undercut belief in God is to try to show believers that they have developed their beliefs poorly in the first place. Of course, I thought Larry was off on this assessment overall, but it is a very complicated discussion in Epistemology (the study of how we know what we know), and the debate can be found here. https://www.recapitulate.org/debates
A case in point of the difficulty of proving a negative is in the field of Cybersecurity. If you scan all of your systems and investigate all of your networks but find nothing, what do you know? All that you know is that you didn’t find anything wrong. But, the truth is that you may not have had the right tools or known what to look for. Plenty of hacks around the world happen under the noses of trained professionals with the latest technology. Thus, proving a negative is hard.
I hope this helps answer the question.