I got 89%ile in XAT but only 83%ile in CAT. Will they consider my XAT score instead of CAT? And how much will I have to score in MICAT 2, say with a decent attempt of the descriptive section?

Your XAT scores would be considered during the final selection if you've applied through both CAT and XAT. Meanwhile if you manage to score above 50 in MICAT then considering your XAT percentile, which is very good, you can easily get through if you perform well in the subsequent selection rounds. For the descriptive test the best attempt was to answer all the 4 questions. The first 2 being of 10 Mark's each and the 3rd and the 4th questions being of 20 and 30 respectively. So if you managed to do atleast the first 3 questions then that would be even better and can assure you a higher chance of final selection. The final selection is based on the following criteria

CAT/XAT percentile = weightage is 20%

MICAT = 30%

Group Discussions/Group Exercise = 20%

PI = 30%