The first rule to success, is to clear your doubts about yourself. CSS is an avalanche test. It tests your strategy as well as your physical limits to with-stand the avalanche fall. Now coming to your case ...
- Cramming will ruin your mental capacities, the new syllabus is designed to discourage crammers. Much of the subject pairs are theory based promoting 'out of the box' thinking and extracting.
- English must be top-notch, try giving TOEFL/IELTS examination to test where you stand. Always improve and polish by reading/listening, you got podcasts, novels, a plethora of resources.

Lastly, AhmedKhattak hit the nail on the head - LUCK, luck + hard work will win you this marathon.

Good Luck!

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"He who has a why to live for, can bear almost any how." (Friedrich Nietzsche)