I went for one of these recently for the same position within the technology and Integration practice. Unfortunately I didnt get an offer, but following is what I experienced and it may help you.
You have a set time for your 2 interviews (1 Partner, 1 Principal) and the group discussion. Around these allocated times it is up to you how you split the day up to answer the essay and construct you case study/presentation.
1. The essay is based on a top level type of question. You have a number to choose from (approx 4) from each of the sub departments within the practice. All that is needed is about 2 A4 pages (so they recommend). I guess they want to see that you can be concise and clear in your writing.
2. For the case study you are given all the information in a handout. It is for you to trake this information and present back using a flip chart. How you approach this is up to you, but I think the vault book or similar will help you in at least deciding on which framework to use. You will need to present you flip chart to the Principal and field questions to back up your statements. You will then get general CV questions
3. The partner interview will be about your CV and experience along with where do you see yourself in 5 years etc
4. As for the group discussion this comes at the end of the day and your experience of this will totally depend on who is also there. You could get lucky and get cooperative candidates or you could get some who are totally dominative.
I hope this helps, I have not given any recommendations as I was not successful and hence what not want to sway your thinking outside of the facts.