1) AGI does not increase stealth.
2) She cannot shoot and move in one turn. Also, the damage from a bow and, most likely, even a musket will not be enough to kill in one turn.
3) She can simply run up to a warrior from the side, without stealth, and take the ability for an instant kill. You can give her a potion of accuracy and speed, by the way.
Added 1 minute ago
Gray text: Ah, if only I had a "Manifestation" scroll (or whatever it was called)...