Untalented
Britney's eyes widened in shock at her spell falling apart, not even reaching the monster. 'Aqua Pila' was one of the most basic offensive water spells a mage could learn and was often used as practice when a person first became a water mage.
As for why she had chosen to use a weak spell such as 'Aqua Pila' was to test the creature's defenses and reaction speed.
If it avoided the water ball, she could threaten it with an overwhelming number of spells to buy time for them to make it to the elevator. If it had tanked the spell, she could test how durable its horrid, flappy skin was.
There was no thought in her mind that the spell would deal any worthwhile damage to the supernatural but what she was aiming for was information.
Britney didn't know many water spells and her mana core possessed much less mana than that of a normal mage. She had gotten this far by learning everything about her opponent and utilizing their weaknesses against them.