why did this "pacifist" go in if she knew the dungeon would spawn a copy of herself that couldn't be beat by any of the examinees? and how strong is fern now if she can overpower via brute force a copy of frieren
I don't think Fern would go the brute force approach.
Frieren mentioned a strategy and it's been stated before that a) Fern is a faster fighter and b) it comes down to more aspects than just brute force such as techniques and skill (although I don't expect this to be of much significance given Fern is Frieren's disciple).
If you add that the clone is outnumbered/real Frieren should be able to keep it at bay, I don't think it's too far fetched for Fern to win this...