I was under the impression that she couldn't be beaten the usual way, since she tends to escape from battle on her own. Well, it would make sense to beat her with magic.
Another user, "TheDisturbedDragon", stated:
I've seen her get over 10k, lvl 500+, and still won. Of course, I am lvl 539... Basically, once she's past lvl 150 or so, fighting her is virtually impossible. She has an absurd dodge chance, so just run away.
That's kind of scary, Venefica seems more manageable.