Actually, Vanadis aren't that all powerful. They are pretty vulnerable to assassination, for one.
This isn't a spoiler because it already happened in the first episode, but the details were cut out. I will tag it anyway and keep it in white text.
Not really a spoiler:
Tigre actually almost killed Elen in their first encounter. The only reason she was able to block his arrows was because he aimed straight at her forehead, twice. Elen deduced where he would aim there because that was where he shot the horses. If Tigre had aimed at her horse or her body, she would have been killed.