How can I crack the IISER aptitude test?

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. 

• Study Theory: You need to be very clear with all the basics in your syllabus. It is of no use to mug up all the high-level books like H C Verma and alike. Just stick to your NCERT books. 
• Once you are clear with the theory of the concepts from your NCERT books, you can look for other reference materials, notes from teachers to brush up on particular sub-topics that you need to be more sure of. You can also make your own notes since writing helps you remember things better and they can serve well in the end for revision purposes.
• When it comes to Mathematics, you can never be prepared enough or solve just the right number of problems. So, make sure to practice as much as you can since that will help you find various techniques to come up with solutions and faster of course. 

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.