It is requested if one has the important quotations in particular for essay paper, idioms and phrasal verbs then share it here.

No one can really predict essay topics of upcoming exam. In 2017 exam not a single topic was from past papers or predicted list.
As you are going to prepare for English Precis & Composition paper, so you will learn a long list of idioms and phrasal verbs which should be enough for essay paper. It totally depends on you that how you use them in paper. Also idioms will also help you in precis and translation.
About quotations, they cannot be used in all topics. It depends upon topic. Surely by using them your paper look more decorative and gives impression that you have sufficient knowledge of the topic but there is no need to forcefully put them in essays. Almost everyone prepares topics related to media, women etc. Quotations can be used in these topics and not only quotations you can also use English translation of Quranic Verses and AHadith as well. Try to use quotations of famous leaders and credible authors only. Practice writing 1 essay every week including outline. Search relevant material. Decorate it with quotations etc if you can and search them while gathering material for particular essay. Plenty of essays are shared by CSP's and senior aspirants on this forum. Just select topic, gather material from (forum, books, news papers, articles and other sources on internet), construct outline and write it accordingly. Concepts should be relevant and paragraphs should be coherent. Give strong arguments and try to convince the examiner. Be moderate in touchy topics like religion and politics. Get it checked from Relevant English teacher who has experience with CSS preparation. That is all you need to know about essay preparation.

Thank you, please can you suggest that from where I can prepare idioms, phrasal verbs and pair of words?

Buy book of English Precis & Composition by Hafiz Kareem Dad Chughatai published by Caravan Publishers. It has everything you need for exam. Do not forget to practice past papers. Papers till 2015 or 2016 are already in book and latest edition might have 2017 paper as well. It has lists of GRE 4000 word vocabulary list, prepositions, idioms, pair of words, phrasal verbs, synonyms, antonyms and one word substitute lists apart from practice exercises of other exam questions. It has compiled data at one place. You will find it very useful.

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WA-TU-IZZU MANTASHA; WATU-ZILLU MANTASHA

For which compulsory subjects, the mentor is must to learn?

I think a good English teacher who have some good experience of teaching css aspirants will be really helpful in your preparation. He can assist you on important fronts like assessing essay and precis writing, and for me I think these two are the backbone of css.

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English in compulsory subjects rest depends upon your optional subjects. For regional languages teacher is required. For general kind of subjects no teacher is required. Subjects like Physics etc are attempted by people who have degree in those subjects. They can take help if required.