The episode where Frieren talks about mana suppression she mentions doing mana exercises, and after 1000 years that's why she has so much. Fern has been training the same way for like 20 years now so she probably has a pretty huge mana pool compared to the average mage. She also only uses basic attack and defense magic, so she's almost like a demon in that she's mastered a single specific talent. In this case that talent is rapid firing dozens of basic attack spells at once.