They may be right but my personal experience has been that a high emphasis was placed on undergraduate sciences such as A&P, chemistry, micro, etc. Fortunately, I had A's in all of those classes so I didn't need to repeat any classes. If I was in your shoes I'd really consider retaking those sciences because you're going to be competing with other students who have earned A's in those classes. It's not something I would rush, I would focus on finishing your BSN off on a strong note and getting as close to all A's as you can. Once you graduate I'd slowly work on retaking some of those science classes perhaps even at a school that has a CRNA program you're considering applying too. I wouldnt rush taking the courses though and I would take them at a pace that would allow you to focus on getting A's.
Applying to CRNA school for myself was kind of like a long journey, there isnt a perfect set path to walk that is going to guarantee you admission into any one program. I personally felt that my application experience was extremely competitive. I had to deal with being let down, being denied, wait listed and eventually accepted to multiple programs. During all of my applications though I continued working on becoming a stronger, smarter and better educated ICU nurse. I seized every opportunity to learn more about anesthesia including listening to podcasts, shadowing, certifications, graduate level classes, and attending the local anesthesia conference specific to my state to network.
Retaking chemistry or microbiology to get a better grade is probably a pain in the *** and I get it, but you should honestly look at it as an opportunity to learn and better understand concepts that you hadnt fully mastered previously.