You need to have great accuracy for that and that would only come with full preparation and revision. Secindly, You wont get accuracy by leaving questions, try to solve all problems initially and where you go wrong solve such questions more to make your weaknesses redundant, once you do that your confidence will boost miles away. Secondly, give maximum mock tests and make sure you dont repeat same mistakes again. once you're done with that you can then choose and select certain questions in final AIMCAT exam to score great percentile wiyhin less amount of time for maximum accuracy. All the best for future endeavors.

Keep Calm and try to identify your mistakes. Are you faulting in speed or accuracy? Is speed, practice more. If accuracy, speed up your calculations by mugging some basics like tables, square roots, cube roots, squares and cubes etc. All the best!!