The spell causes all of the magical power in a mage's body to go out of control, harming the mage. Rei possesses so much magical power that it can't be measured, meaning Dominator would cause far more harm to her than it would to anyone else. As I recall, Manaria wasn't aware that Rei was so powerful, so she may not have expected to hurt Rei so badly.
Ohh... That means she could've just killed Prince Rod in a very painful way on their fight using that spell...
Also, I agree that Manaria is stupidly strong because she wasn't supposed to be a boss you can win against, or even fight her. There are tons of these characters on videogames, sometimes the mission is just survive for a specified amount of time/turns or even worse, just lose the fight no matter what (and being rescued for some plot armor, depends on the game)-
Why does someone like her even exist? Well, remember this was just a otome game, don't think too hard about it...