Mortality seems to be the big overarching "theme" of the series, and how despite humans having the most fragile and short form of it they seem to make the most of it. In contrast to the elves and demons (and lesser extent dwarves, but they don't get much focus) who have all the time in the world, yet treat it so lightly that they let decades or centuries just slip by while only improving passively.

Fern is narrowing the gap between her and Frieren basically every time we see her fight. Its still a massive gap, but its still getting smaller despite the extremely short amount of time relative to Frieren's life Fern has been learning from her.

And that's someone who isn't really all that interested in magic herself, just is innately good at it and has a good teacher.