I attended an ACN final round assessment for IT quite recently. It was very difficult compared to others I have faced. You get about 15 mins to read a long case study and then make notes and then give a sturctured reply.
They will ask you to summarise the most important challenges the company faces such as poor business processes, IT software failure and poor systems architecture, low staff moral ect. Then they will ask you to name the most important, then they will argue that its wrong and another is more important.
Remember they are testing for decisiveness and stick to your guns and defnd your decision. do not be swayed because its very intimidating when they try to prove you wrong.
Oh... and in the group work you may want to check that all the candidates have the same information. In mine we had different peoples opinions for the case study.
I wish I could tell you it will be easy, but it will be difficult for you to prepare, other than to keep in mind what I have said. ACN will either like you or not, its all about fit and how they think you will blend in. You get that impression from the start.
I didn't enjoy my assessment experience, i found it unecessarily pressured and not friendly at all. I am not saying you have anything to worry about, you may have a totally different opinion, but when I got offered a position I turned it down because I just didn't like the atmosphere or culture.
ACN is a very personal choice, for me it just wasn't the right one. As you will understand if you are a frequent visitor of this forum, AC is like marmite, you either love it or hate it!
I hope this helps and I wish you the very best of luck! Let us know how you get on.