Aura hypnosis ability is based on mana, she just compares her mana power to her opponent, and the weaker one loses all free will. Frieren is one of the most powerful wizard in the world... so she got cocky, but she didn't know Aura had another ability:
Serena can't remember why she's collecting Hypno semen. That... is what she's doing, right? She's filled up her condoms, and her belly - more yet pulsing into her womb... Yes. Yes, this makes sense. She must need this semen; she needs it. (everyday2)