Frieren refers to Serie, several times, as a War Mage. In fact, it's made clear that the reason why Serie and Flamme couldn't defeat the Demon King, regardless of how ridiculously OP they are (for christ's sake, Flamme is so strong she managed to cast spells that have lasted over a thousand years), is because they couldn't fathom an Era of Peace.
Frieren is a Mage of Peace. It's what Flamme even says. Flamme is aware that Frieren loves Magic for Magic's sake and that she isn't warlike - even though she's capable of waging a silent war for a thousand years just to get revenge for the demons who killed her people.
Re-reading the series provides so much contextual clues early on and then they all start to make sense the more you go forward; everything, or nearly, is foreshadowed in one form or another.