Damn. If you think about it, she could have defeated her in almost any other way. Instead, Frieren prefer to wait until Aura tried to submit her and then have Aura kill herself...
For an emotionless/apathetic being blessed/cursed with eternal youth: that's pretty fucking vindictive and savage.
Holy shit, this is literally as bad as the Han Shot First Controversy: the whole point is to show much of a POS character evolves and develops in a better person. If you take away his "POSness" this early and at this stage of his development, it literally lessons the value in the character. Who...
....Why can't they meet every night or every other night? Cause he's in a traveling circus? OK, then why not change the meeting location with the circuses' travels? Why not have a giant portable bowel of sea-water for her to rehydrate and recuperate in while she masters the magic to temporarily...