Well done getting into the final round!
Now, first thing's first: Check with HR as to the format. It may be fit + behavioral, or just one of them. If may have market sizing or it may not. It may be with Partners or with Managers/Vice Presidents.
That being said, in reality, you need to be prepared for anything and everything. Here are some tips that may help.
Fixing your Weaknesses
They have made a note of areas you showed strengths/weaknesses in. Look for them to test you weaknesses to see if they're 1) A fluk or 2) Workable.
So, focus on improving in areas you know you underperformed in.
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.