This depends on what you mean by "approach."
One possible answer is that Diamond takes a geographical approach. Diamond discusses the main issues of this book by looking at the impact that geography has had on various societies of the world. He finds answers in geography, not in race or culture. If "approach" means what factors he finds important, then his approach is geographical.
Another possible answer is that his approach is anecdotal. Here, "approach" means the kind of evidence he uses. Because of the nature of the questions he is addressing, Diamond can only use anecdotes from history. He cannot do any sort of a systematica, statistical survey of evidence. Instead, all he can do is provide anecdotes. One can criticize this approach because it allows the author to "cherry pick" and use only anecdotes that support his position.