Sure. Pleasure is all mine.
I will highly recommend you to buy book English Grammar & Composition) by Hafiz Kareem Dad Chughatai (1st position holder of CE-14 written and overall 2nd) published by Caravan publishers.
He has written his book according to new syllabus. 4000 words GRE vocabulary list is also included in the book. He has covered everything in the book i.e precis, comprehension, vocabulary, tenses, narration, prepositions, punctuation, translation, phrasal verbs, sentence correction, grouping of words, pair of words, idioms, narration, proverbs, one word substitute etc. It does not have change of voice because it is not required but I have prepared it as well because I write many of my sentences in passive voice to make it different from others. I have been practicing it since 2012 and I can tackle it. There is no need for you to learn it for CSS so, book is enough. Apart from covering all the topics he has included practice exercises and solution of past papers as well. He has provided solution of all the available past papers. Some are fully available and some are partially. I have bought around 4 books and only this book is worth buying so, do not waste your money on other books. It is a complete package.

About clearing CSS exam by an average student, your friends and your brother in law is totally wrong as I know many people who have cleared CSS and are average students. Academic performance does not make one average, poor or brilliant. One might be studying something in which he or she does not have aptitude. One needs to identify his/ her aptitude, strengths and weaknesses. Like I can excellently tackle general theory subjects in which I can reproduce the topic in my own words. I can do analysis in my own words. I am not good at cramming so, I try to avoid those subjects. So, identify your strengths.
No one can tell who can clear CSS and who cannot. Your friends and BIL is making comments that every other person makes after failure. One can totally do it. Do not listen to them. Focus on your goal and stay motivated and determined. Time taken to prepare for exam varies from candidate to candidate. Usually 6 months to 3 years can be taken to prepare. Average student who already have aptitude and good command on English can easily do it within 6 months. So, As I have suggested above, it is time for eat-study-sleep routine and you are required to cut off from social dealings. So, do not waste any time further. Just start studying. For keeping your mind fresh you can get entertained by watch movies, seasons, reading books etc for a little time after get exhausted from studies. Otherwise bury your self in books and try to keep social contacts as minimum as possible. I know a girl who had prepared for 4 months only and got allocated in PAS in 2011 batch. She was totally socially cut off. She had rented a cubical room in hostel and started preparing. She was just consulting books. No mobile, no internet. She used to study 15-18 hours. She was not a brilliant student in her previous exams. Everyone can do it man. Only time taken to prepare differs. If you have no age issue, you can avoid this attempt otherwise take chance and try at least once. May be you too get lucky and become a part of most prestigious service in the country. Never ever let any body to dis-heart or discourage you. Choose subjects smartly. Gather resources both soft & hard. Start studying. Keep praying. Give your best rest leave on Allah Subhan Wa TAllah. Even if you fail, there are vital chances of you getting a better job. After preparing CSS, you have so much knowledge which can take you far in your career. Getting other FPSC and PPSC jobs will be easy for you. Brilliant students do not always have brilliant minds. Sometimes the thing makes them brilliant is their stubbornness and they can easily study for many hours a day. So, do not worry and start studying.

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WA-TU-IZZU MANTASHA; WATU-ZILLU MANTASHA