Go through the promotion paper and prioritise the topics according to the number of marks lost. Start by revising the topic with the most number of marks lost. For every mistake made in the promo, practice at least 3 more questions of similar or higher level of difficulty.
Focus on brushing up Calculus, Sequence and Series, Vectors and Complex Numbers. These four major modules account for more than 60% of marks in A level.
For students who failed their promotion paper and need to retake a supplementary paper, engage a good tutor to shorten the learning cycle.