I haven't done Bain's written case, but I did do AT Kearney's 2nd round case, which is written.
In both cases you have roughly an hour to review material, then have give a 10-15 presentation, followed by 10-15 minutes of questions.
I've heard, though I'm not sure, that Bain's cases tend to be really long. My AT Kearney case was about 6 pages, and 3 or so more with figures/exhibits. Not insurmountable. AT Kearney's case also had roughly 6 questions on it that, if you thought about it, helped to guide you through the insights they wanted you to see.
If the case is really long like I've heard Bain can be, the strategy is basically: 1) Read the executive summary, 2) Go through the exhibits / figures / charts, 3) See if you have answered the question / have a hypothesis and supporting info. If the answer to 3 is yes, then I'd start with your presentation then; once you have that in a skeleton form,t hen perhaps scan the text to see if there are details you'd want to include; if the answer to 3 is no or youre not sure, perhaps scan the text first (before starting presentation) to see if that answers it.
Regardless of how much material is, some things to remember:
1) State your big conclusion at the beginning
2) Make sure you answer all the questions posed (how you do this is personal preference and depends on how many questions there are... you may want to answer em up front, you may do it at the end, or you may integrate as you go)
3) Limit yourself to 3-4 slides. And make them simple. Most of your presentation should be talking points. Slides should be clear, organized, basically client-friendly
4) Don't be surprised if they stop you mid-presentation and throw some doubt or new information at you ; just like an actual case, they're going to want to see you think on your feet. If you need a second to consider that, ask for it. Good cop/bad cop routines are common too.
5) Confident and concise, just like any case.
6) Leave notes / sticky notes / highlight key figures, sections, and areas from the case. In some cases you may want to refer to it (usually answering a question after your presentation).
Victor Cheng does have a youtube video up about the Bain written case, IIRC
If you have any more questions, let me know. Cheers, GL. I might actually be able to 'give' you a written case if you hit me up.