#1 Fix your weaknesses
#2 Be prepared for anything (build your fleibility/adaptability)
Some reading for the case: https://www.preplounge.com/en/articles/how-to-shift-your-mindset-to-ace-the-case
Some reading for fit: https://www.preplounge.com/en/articles/tell-me-about-yourself-interview-question
You should be more prepared for a pretty in-depth fit interview. I highly recommend you expect "loose" or "random" questions and practice this scenario with a coach. They are less likely to just ask you 20 "tell me about a time" questions. They may have a bit more of a chat, ask you to brainstorm, ask things like "How would you setup x type of project", etc. etc. You need to prepare to be flexible here!
Broader topics/case variability
In terms of being MECE, try out a wide range of "unique" questions to test your MECE structure!
For example, if you were a farmer and had to pick between buying a cow, chicken, or pig, how would you think about which to buy?
Or, if you were a thief, which store between a, b, c would you rob?
Practice breaking these down into MECE structures so that the concept really hits home.