You need to score more than 100-110 atleast to secure a seat at the IISER.You can also get admission over there with your jee advance rank.If your jee advance rank is within 10000 you can apply.And for the exam you need to score good.The exam is not too difficult if all your concepts are clear.

Regarding your touch with maths,try to revise all the topics of class 11 and 12.You can follow some of the books like differential calculus by Amit M Agarwal and many more books of different topis of Arihant.

Best of luck.Study hard.You will definitely do well.