Hey Siddhant, Hope you are doing good right now. So coming back to your question as you said you got 8 CGPA in class X and 70% in class XII. To tell you if you are looking to get admission in a good B-School definitely your past academic records matters a lot. But their are also some top B-Schools which don't put that much weightage towards your past academic records. If you are willing to give CAT 2020 then make your fundas very clear, get your queries together and get it clarified as early as possible and yes secret to success is to practice a lot of practice and determination.  If you get a good score in CAT then you can get admission into new IIM and some other top colleges as well.

If you ask me then I would suggest you to give other exams too like XAT,MAT,NMAT,CMAT ,IIFT and SNAP. All these tests are important and XAT being the bit difficult but XLRI you can target through that. Past academic won't matter that much but it can haunt you but the thing is you should know the strategy to tackle the questions on your low performance in cCass X and Class XII. You would be through if you can make it.

Best of luck and perform your best.