[quote=@Cio]
I can give her water form a glaring weakness too though, maybe not fire since that doesn't quite make sense but something else.
I thought she would be weak to magic in general because she's a mythical creature, so a mage would have a easy time defeating her even underwater.
[/quote]

It may be a little weird but most mythical creatures have odd habits that expose a weakness. Like, if you think about it it's really random that vampires can only enter houses when invited and werewolves can only be killed by silver weapons. Strange traditions are very important to myth!
In the CS you mentioned that she was caught at first because someone threw a rock in her lake. Maybe Aava compulsively surfaces if the water is disturbed? Whether due to an intangible magical force of will or her instincts dictate as much on a constant hunt for prey. I hope that made sense.