Her dragon form is probably not level 8, her human form is most likely weaker than her dragon form.
There is another dragon (forgot the name) on the island who is also around level 7 or so and honestly that is more questionable than her being level 8.
To add to that, she clearly his consuming Magister Atusa source when she kills her, so she probably is going to grow stronger as the game advance because of it, just like how Braccus Rex grew stronger (and crazier) from devouring his enemies source.
