If your goal is to get into an allopathic school, then take upper level courses and ace them. If your goal is to get into osteopathic schools, then retake the classes than you got a C or lower in.
Allopathic schools will average all your grades, including retakes. Osteopathic schools only use your most recent grade in their calculation. So if you made a C the first time, then a B+, it's the B+ that counts. Likewise, if you made a C the first time, then a C- the second time, it's the C- that counts.
I would just take 20+ science credits in the coming year, and bump up your GPA. Or you can do a combination of retaking a few courses and taking upper level courses. It's up to you. But, I don't recommend doing a masters, as none of those grades will change your GPA. They get added into a seperate GPA number.