zrobić co w czyjejś mocy in English:

Just do your best.

It doesn't matter if you don't win, just try to do your best.

All that matters in the exam is to do your best

You have my trust, so do your best.

Don't worry about your exam, you can only do your best!

You can do your best to buy natural products and to buy from companies that you have always trusted, but that is not always possible.