Here are my inputs to your questions:
1) To start from scrap the best book is Manhattan?Yes, get the complete set of Manhattan Strategy guides 5th edition. Make sure to read their book on "GMAT Roadmap", it will guide your way until the last day of GMAT.
2) I feel like i am missing some tactics for GMAT question, because i am known to basic concepts of quant but still i lacking in solving GMAT problems so where i need to improve? Take a GMATPrep test or MGMAT free test to gauge where you stand and identify areas weaknesses. Make sure you are thourough with all or most of the content from MGMAT quant/verbal books. For CR you can refer Powerscore bible (optional). both MGMAT and powerscore are almost equivalent in quality/content.
3) If i am planning to prepare from scrap can i take the practicing test after i complete the full practice of 3 Months for verbal and Quants.You can start doing the tests once you are through with basics and have covered 70 to 80% of the MGMAT books content (by next 1.5 to 2 months). Plan your practice tests, such a way that you start your tests 5-6 weeks before actual GMAT (considering 1 full practice test every week-end) 4-6 MGMAT & 2 GMATPREP (minimum)
4) IR section is really hard, is there any recommended materials for IR?Go through OG13 IR problems and MGMAT IR. It should be enough for IR
Let me know if you need any more info.
Tough 700+ Level RCs: Passage1 | Passage2 | Passage3 | Passage4 | Passage5 | Passage6 | Passage7
Reading Comprehension notes: Click here
PowerScore CR Bible - Official Guide 13 Questions Set Mapped: Click here