For these exams your numerical aptitude, reasoning, English, GK should be good along with engineering subjects, speed of solving objectives questions matters most. And most of the objectives questions are repetitive from previous year question papers' so practice them, you will surely qualify.

Which diploma exam are you talking about? As far as I know there are the polytechnic courses but they are state dependent and the process is separate for each state. If that is what you are looking for, can you tell me which state you belong to and I shall get back with the relevant details for you to apply and pursure the course.