Yes, CUSAT examination is difficult. But if you prepare the syllabus well, it’s not difficult to score. The examination covers more questions from Maths. The questions are usually basic. So, it will be better if you focus on Maths more while preparing. Solve the basic questions from each chapter.
For the Physics section, most of the questions are based on concepts. Most of the numerical is formula-based.
NCERT books will be enough for Chemistry.
I have a few friends who are studying at CUSAT and according to them, yes the entrance examination for the institute can be tough. However, if you prepare well enough, you can easily score a good percentile. In CUSAT the examination is lengthy and at the same time tricky, so speed and accuracy are of foremost importance.
It will be better if you go through the question papers from the previous year. This will give you an idea about the examination pattern. Also, with luck, you will find some questions repeating themselves exactly from the previous year’s sets.