I have scored 91% in 12th cbse board 2k18 and literally speaking, i did so in the last 3–4 months. So i think i am quiet eligible to answer this. And also this answer is gonna be long.
Scoring 80–90 % in boards is easy if you are consistent in your classes and every topics and concepts are cleared to you. But 95% and above is somewhat difficult. If you study the last 10 years papers, almost 70–80% questions are those in which you don’t have to use mind, they are merely based on facts, formulas and concepts only. Then to attempt the 100% paper correctly you have to develope the analysing part and higher order thinking capacity.