Hi Akshay!

You have a pretty decent score in both your SSC and BE but there is downhill in HSC. Since you have not mentioned the category you belong to, I'll assume that you fall in the general category and provide you the data accordingly.

First of all, the basic eligibility for CAT is 50% in your graduation and you are in the safe harbor. Getting into an IIM is a different story though. Each IIMs have their own admission criteria and vary accordingly, and the weight given to CAT percentile is also different. Some colleges like IIM Bangalore depend heavily on your academics, which in your case is dented in HSC and stand a low chance, but other IIMs like IIM-Calcutta go easy on the academics and weigh heavily on the CAT and this becomes your hunting ground. Thus, it becomes important for you to jot down the IIMs you are eligible for and aim for the particular CAT score accordingly. 

Since nothing can be done about the HSC score right now, it becomes rather important for you to score as high as possible in your CAT exam. Assuming you get somewhere around 99 percentile, there is a very high chance of you securing a seat given that you do your PI-WAT very well. With a score like 99 percentile, you will certainly get calls from all the new IIMs, baby IIMs, and sometimes even the old IIMs like IIM Indore and IIM Kozhikode, but getting a call from the holy trinity (IIM ABC) would need a score of more than 99.6 percentile. I personally know students who haven't got a call at 99.5 percentile and I assume you know the cutthroat competition by now.

That being said, once you get a call from the IIMs, another obstacle to cross is the PI-WAT. Again, it depends upon different IIMs. Some IIMs like IIM Bangalore and IIM Indore give very less weight for your CAT percentile ( a mere 25%) and others like IIM Udaipur give 55% to your CAT score while calculating your composite score for the final admission. Thus scoring high only in CAT doesn't guarantee a seat and the real game begins only after that. 

Good luck belling the CAT! Feel free to drop in your queries.

Find out about IIM admission criteria here - https://bschool.careers360.com/articles/iim-admission-criteria