Priya, PESSAT is not extraordinarily difficult to crack it. Preparation can be done alongside other entrance exams.

For example if you are preparing for JEE Main then its preparation will be sufficient for PESSAT also. In addition, there is an English section also, so just focus on preparing that in addition.

Securing CS does not depend n your marks, but rather your rank.  For securing that branch, you need to get your rank within the range of top 900. Beyond that it is difficult. But again it depends on the number of students who appeared for the exam and as well as the difficulty of the paper.

Feel free to ask if you have any more queries.

Hope this helps!

All the best!