Negotiation - fairly easy, the key is to come to an outcome. The funniest thing is it is often the most insignificant agenda item that can prolong the exercise. For example, if I remember correctly, as a part of a director's compensation the opposing team wanted a Ferrari but we were prepared to go as far as a Toyota. It was kinda funny with the grad rec giggling along but remember to share your thoughts, take space in the process and more importantly, if members of your team are hesitant, invite them to share their opinions!
Written exercise - the biggest mistake I made was not showing my working. There was a significant math element (very basic don't worry although note that no calculators are allowed) and I tend to just do them in my head and my answers were basically numbers with no explanations of how I got there. The grad team would likely want to see how you got to your answers so no matter how easy the calculations are, make sure you write it ALL down so you can take the grad rec through your thinking process. Timing is KEY - it is more important to address 5-6 issues in depth rather than 10+ superficially. I would also ask for a highlighter so you can get through the material quickly.
Interview - like the written exercise, you are given a piece of news or contract (I was given the latter) and this is where I totally bombed it as I kept answering questions based on memory instead of looking at the damn paper. Also, the partner interviewing me asked some really challenging questions which I think can be overcome if you interpret the document to Mac's clients and the commercial environment. I am afraid I can't remember the exact type of questions asked but they were rather technical from a commercial sense - like if an employee is made redundant based on the contractual nuances I was given, what sort of role would Mac play, etc. The motivational questions went super well so make sure to know what you wrote on your application but do not regurgitate the same reasons for why Mac etc.
In general - You will also have the opportunity to have lunch with trainees one-on-one so remember to ask questions, about their trainee journey (a lot of them happen to be paralegals at the firm who were eventually brought on as trainees) and as hard as it may be, have fun! If you practice enough of the case studies, it can be really exciting to tackle the assessment case studies with the adrenaline going. Also, grad rec gives off very no-bullshit vibes so don't try and be overly friendly and act interested in them because they honestly couldn't care less but are genuinely interested in you as a talent. It is a very VERY academic firm so I would focus on reading their blogs and articles prior to the assessment which is also likely to help you in the partner interview because there are some really intelligent people interviewing you!
Really hope this helps, best of luck and I am sure you'll smash it!