Based on the resources that you've mentioned, looks like you did not have any interviews with great partners.
1) The first way is to find the right partners. Your preparation is more about quality rather than quantity:
- Don't count the cases you've done yourself
- Don't count the cases you've done from the casebooks
- Count the cases you've done with experienced candidates (50+ cases solved / passed the first round) who can give you a real case and a good feedback.
2) Another way is to have a good coach. If you take a good coach, you can make it in 20-35 cases. The coach will give you the right knowledge. Partners will help you integrate the skill. At the end of the day, it's just a skill that can be trained.
Why do you even need partners and coaches? When you are stressed during the case interview and out of energy having the 3rd case in a row, 90% of your questions and reactions should be fully automatic. Otherwise, it's extremely hard to solve the case, trying in find out what you've memorized from v.Cheng book.