Any tips on solving DHLC business cases? Thank you!

I guess my tips are rather general, but I think the first and best advice I can give you is to practice as much as you can, ideally together with a case partner. When preparing,  I also used cases from PrepLounge case library.

Second, sleep enough time before the interview day. Recruiting day consists of several interview rounds, so you will need enough energy. 

Third, while solving a case, reason out loud. This will help you, in case you are going in a wrong direction with your solution, to receive guiding question from the interviewer at an early stage.

Last but not least, while solving a case make your notes and calculations in a tidy way. At some point you might need to restructure or recalculate some parts of your solution, and it will be easier with well-structured notes.