Retaking subjects depend on how your university calculates CGPA. Some schools practice "append method", where the amount of pointers you get is divided with the total credits you've attempted. This means the more time you repeat, the harder for you to score.

Other schools use "replace method", where your pointers is divided with the total credits acquired. This means you can repeat and replace old grades as you wish and the only drawback is time.

There are several issues with your intent:

1. If I'm not mistaken, PTPTN only consider 1st class with graduate on time for the exemption. Please check on this. If confirmed, it means you have to retake 4 subjects without adding a semester to qualify.

2. Most schools take the latest grade. Meaning if you repeat a C+ subject, but get a C later on, you will use the C and the C+ is burn.

3. Some schools don't even allow retaking a non-failed subjects. And those that do, have an upper limit. Meaning you can only repeat C+ subjects, but not B-.