Hello, landlord: once I participated in a math contest, I attacked the difficult questions of cmo and imo. At that time, the progress was quite fast. I read the book Towards IMO, which is very good and the difficulty of the topic meets your requirements. There is also an all-Russian Mathematical Olympiad for middle school students (1993~2006), which is not bad. You can choose both books. I have seen and done it myself, and recommend the former (for IMO). Hope to adopt, thank you!