The admission criteria at all the IIMs changes every year. One year you may get admission with relatively low scores but the next year you might now get in even with a high score.
It all depends on the panel of judges and your luck. So my recommendation is that you should not depend too much on your 10 and 12 marks.
Go for the CAT and score 99 percentile. Chances are you will get admission even with low 10 and 12 marks. But without CAT you need better marks in 10 and 12 to make it into any IIMs.