To achieve an A* in your synoptic essays you must show breadth of knowledge as well as depth. What examiners want to see when they mark your answers is that you have a thorough understanding of the concepts throughout the course, and for an A* answer, that you are able to be insightful throughout your argument. This means they want you to be able to see how the different aspects of the course are related and be able to demonstrate your knowledge with relevant case study evidence.
The mistake many students make when using case studies in their answers is 'case study dumping', where irrelevant details are given to support a point. Ideally, to achieve an A*, parts of case studies should be used specifically (or 'cherry-picked') to demonstrate a point you are trying to make. So for example, if you were making a point about environmental issues that cities in the developing world face you may use Mumbai's slum, Dharavi as an example. You may know tonnes about this case study, but here you would only mention details such as the number of people per latrine and the waste running on the streets, but it would be irrelevant to mention the attempt to regenerate the area by rehabilitating residents to high-rise buildings. Keeping the case studies relevant helps focus your essay, setting the foundations for an A*.
In order to keep your argument coherent you must also have a good essay structure. Having a clearly laid out argument through your essay will help you demonstrate the higher level skills needed to achieve an A*. So, in the introduction you may want to lay out your argument, demonstrating that you undertand the question. The main body of your essay is where this argument will be carried out, and you can show off your knowledge of the course, and the conclusion should tie together the different points you've made. To keep the essay focussed, you may want each paragraph in the main body to end with a sentence that relates it back to your argument and therefore the question. These sentences are often where you can make those insightful A* statements that show 'flair'. If not here, then the conclusion is another place you can show some flair that demonstrates your confidence in the course content and ability to handle it critically. And you don't need to go crazy with this either, to achieve an A* you'd only need to include this once or twice for the examiner to see you are capable of it and award you the A*.
If you really want that A* in those exam essays then a lot of it comes down to ensuring you make the connections between different aspects of your course as you go along. This way you're thinking about the course synoptically from the get-go and when it comes to it you'll find that writing a good synoptic essay comes more easily.