@Kaarme
Sorry, I was on my phone and didn't elaborate.
You say Amanda didn't make a mistake, but you're basing this on an assumption that isn't even supported in the story as we've seen so far.
Why should an A rank not make such a mistake?
Chapter 14
Page 5: The guild master questions Fran about the demon's mana crystal. Fran says she broke it when she killed the demon, and the crystal disappeared. That's essentially what Amanda did just now: she killed the spider, and broke the mana crystal as a result.
Page 24: That didn't stop guild master from promoting her 2 ranks from F to D. He even admits he wanted to promote her "to C or even higher", since she single-handedly defeated a rank B target. Her destroying a mana crystal by mistake is not what stopped her from reaching B rank, it was petty politics that did.
Heck, the only reason she even got to defeat a B rank as a mere F rank in the first place, is because she disregarded orders, and Donadrond (the guild's examiner) scolded her for that (chapter 13, page 17). Still got her double promotion right afterwards.
Chapter 16
Page 28: Fran is to investigate the spider dungeon. Amanda, who listened in, literally sends the guild master's door flying. Was it on purpose or an accident? It doesn't matter. Either way, she wasn't supposed to run the guild master's door in.
Page 29: The guild master face palms. That's not the reaction of someone who sees an A rank adventure break a door for the first time. She's likely done something similar before. She's still A rank despite making such a "basic mistake".
Besides, why would Amanda even destroy the mana crystal on purpose? She's got the title of "Guardian of Children" (Chapter 16, page 22), which is a title given by the Gods. Would a guardian purposefully prevent someone from getting stronger? For what reason?
I'm not saying that it's impossible, but at this point you have no evidence, and your reasoning has just been disproved.