IISER Aptitude Test is basically a screening test. The eligibility criteria for attending the test itself is kept above a certain standard that only the top students from each school board are eligible to apply. What the admission committee needs to do then is to conduct a test which will filter the best among the best. The test doesn’t have to be super hard as IIT JEE, because the eligibility criteria only makes the top fraction of students in the country eligible.

I have seen the question papers of 2010 and 2011 and they were definitely not as hard as the IIT JEE papers of the respective years. I don’t know how much the difficulty level of the test has increased over the years, but I believe it wouldn’t be as hard as the JEEAdvanced.

All the best for the test. Prepare well.