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The most difficult ATPL subjects
Instrumentation: A lot of concepts and details; a relatively small number of questions pertain to the same issue. Many quite abstractive problems are difficult to be referred to the existing knowledge. Moreover, the subject involves a relatively high number of questions.
Airframes: A subject that serves as an introduction into the world of airplane systems.
Meteorology: The most massive ATPL subject. It's worth to learn this subject with understanding, as it makes memorising numerous issues easier. Bear in mind, that majority of issues are more or less directly interrelated, thus understanding one issue makes it easier to comprehend others.
Radio Navigation: A lot of concepts and details one must simply learn by heart.
General Navigation: A subject many students wrongly assume to be highly difficult. It comprises a theoretical part and a practical part. The theoretical part involves a dozen or so topics one ought to master. The practical part involves problems that constitute a major part of the study material. They are all based on around 10 solution schemes. When solving them, pay attention to details, e.g., the symbols +/-. Although you know how to solve a given problem, getting the symbol wrong gives you a completely different result. While sitting the exam, you've got about 2 minutes (on average) to solve one problem. For that reason, try to gradually reduce the time dedicated to solving the problems to prepare for the exam. Solving Navigation problems will be more time-consuming than solving problems of other subjects, especially at the first time. Don't get discouraged it's perfectly normal.
Principles of flight: the second most massive subject after Meteorology. When studying this subject, focus on learning with understanding. Dedicate some more time to visualise a phenomenon you're studying and comprehend it. In this subject, many questions pertain to the very same issue. You should learn formulas you can find in study materials and charts presenting various interrelations by heart. Knowing these formulas is vital for answering many questions. To this end, I recommend studying while holding a model airplane in your hand, which in many cases will help you visualise phenomena that occur around the aircraft.