A few of my friends have taken the IISER aptitude test and according to them, the difficulty level of the questions asked is equal to or less than JEE Mains level. They are mostly conceptual and there will be very few numerical questions. 

IISER aims to test your conceptual understanding rather than your problem-solving skills. 

• Maths - If you are preparing for JEE Advanced, then the questions usually asked in the maths section for the IISER aptitude test will be quite easy for you. 
• Physics - You can easily solve most of the questions if your basics are clear. The question also won’t require you to do lengthy calculations. Physics questions are usually at the JEE Mains level.
• Chemistry - For Chemistry too, the questions will test your conceptual clarity. 
• Biology - Reading NCERT textbooks will be sufficient to score well in Biology. 

So, to score well in the IISER aptitude test, thoroughly read your 11th and 12th textbooks.