I've not experienced CMS' AC but generally the case studies can be quite time pressured if you have to read a handful before you present/answer questions. So for now I'd advise on practicing your ability to digest information quickly by maybe reading articles and quickly summarising them in structured manner in however minutes you would have during the assessment.
The format is always dependent on the firm but most start with a mini presentation and then they will probe you further on whatever you've picked out. My advice would be to listen to their questions carefully because I've messed up before where I misheard something and answered something completely different. And they didn't correct me either!
Finally, don't panic before/during the AC! You've made it this far, it'd really be a shame to fall at the final hurdle because of something like this (having said that, it does happen so don't think you're alone). Think of it as an amazing opportunity to either massively learn from or to obviously get a scheme - both are fab!! good luck!!!