I can buy it, she was able to level up through a combination of 1) advise from the strongest being in the universe, and 2) a scientific experiment with an extremely low chance of success. I don't think it's a big stretch for her to leap from A class to high level S class based on those two factors.
I know we're all dying for Akari to arrive but I'm loving this fight. It's like in One Punch Man, how the fights are the best BEFORE Saitama arrives and ruins them by being unbeatable. Goldilocks is having one of those life-defining battles against impossible odds that Saitama would envy.