I teach this - what exam board are you doing - OCR or AQA? There is a difference in style of questions and the prescriptive nature of the respective mark schemes.
Tbh, I would revise as despite it being critical thinking, OCR in particular is very prescriptive in the way they answer - they do at least need to know how to answer the questions. Also, ensure they know the technical things like the names of the 40 fallacies they need to know, the 4 criteria for judging analogies and so on...
Interestingly, at work we have a chart which was published (not sure by whom, but I know it in part used the 2008 Score report) which ranked the difficulty of the different 'A' level subjects. Critical Thinking has traditionally been dismissed as a bit of a dossy subject, but this report said that actually it is quite difficult to pass and statistically students are more likely to fail this than other courses (although I do accept that the score report is flawed). Indeed, if you're planning on studying philosophy at Oxford Uni, they actually say Critical Thinking may provide better preparation for their degree than doing philosophy 'A' level.
However, I do also think there is an element that bright students often naturally click with the subject quite easily, but weaker students REALLY struggle with it. That's just my experience though!
Both OCR and AQA do books which give you the basics. Otherwise, John Butterworth has a critical thinking book which is quite good. HTH