To your first point, I recommend you follow all the advice relating to casing that is out there - if you can do a case for MBB, you can do a case for Dalberg. This entire site and many others are dedicated to this very question - i recommend you buy case books, search Q&A, pratice with other PrepLoungers to answer your massively broad first question.
In regards to Dalberg, as you're learning how to nail cases, make sure you focus on cases relating to what Dalberg actually does, key words including:
- Emerging markets
You should be VERY familiar with the challenges posed by the African economic, political, and social landscape when it comes to completely ANY case type. I have a few cases I could give you if you're interested.