is it tough to get through IISER aptitude test??

Hii Lakshmi!
Nothing is tough when you are prepared. So, don't think about such things and just focus on your preparation.  Focus on basics of all the subjects and solve previous year question papers as well as question banks. This will help you get an idea of the question paper and the type of questions asked. So, you'll be prepared well. Just have faith in yourself and do your best. Nothing is tough.
Hope this helps.
Good luck!