what is the possible cutt off marks for RBI Assistant exams,and what is the smartest ways to prepare for the exam in less time?

There are no such predictions exist. It depends upon how many vacancies are available at the office you're applying for. Study whole thing and without thinking about cut off just focus on answering correctly for each question. There are 6 lakh + people in India who are appearing for this exam. Competition is about giving correct answers in minimum time. Just focus on that. Don't count marks, count for accuracy rate. Other things will work out. Practice is the only key. Yet your goal should be answering 80+ questions correctly at Pre level. All the best.