Preparation for the IISER Aptitude test will not ask for anything more than your preparation of Boards. The syllabus includes all that you studied in class 11 and 12 and for exams like JEE (mains and advanced), AIPMT, NEST. You need to keep some pointers in mind while you plan out your strategy of preparation.
Hence, in general study and practice all the subjects- Physics, Chemistry, Mathematics, Biology and be thorough. It is okay if you did not have either Maths or Biology- don't attempt the questions you are not sure of, that will lead to negative marking. You will need to get more than 35 questions correct out of 60 in order to secure a seat at the institute.