This enemy can be an annoyance with that poisoning attack. Even if you can hit the 3 Evade target reliably, that's still a loss of actions as it chases you.

The ability gain conditions cause it to stay on an Ancient location with clues, which you can potentially use to leave it behind. You won't be able to clear that location without confronting this enemy again, but it will be easier to clear all the others. You could deal with the location it guards last, or even just consider it abandoned.

Or maybe you just take the vengeance point.