First of all, if you are a science and engineering major, the difficulty of cross-majors is much smaller than that of general majors, but the difficulty is always greater than that of general majors, so you should be prepared.
The second problem is that the difficulty of specialized courses varies from school to school. As long as the real questions of previous years are done to death, you will understand that the problem is basically not big. Besides, I don't know if you find it difficult to study in high school. There is no problem in high school, and there will be no setbacks in the postgraduate entrance examination. Postgraduate entrance examination is easier than college entrance examination. I suggest you don't apply for the postgraduate remedial class.
The third problem is to decide the major before deciding the school. Famous schools with popular majors must be difficult to test. Generally speaking, the examination questions will be more difficult than ordinary schools, and the marking will be stricter.
I hope it helps you. You can ask again if you don't understand anything.