I think Os'Gabella could take out an entire army, if she felt like it. She cannot be killed, is the worlds best hunter, and is incredibly strong, so given enough time she could stab everyone without even flinching as they attacked her. The fact that she could teleport them to anyplace she felt like, or summon
anything from
anywhere would make the job even faster. The only reason she doesn't do things like this is because she does not need to, and she probably realized it would draw attention to her and make her life more difficult. Os'Gabella is one of my favorite "villains" ever.
Os'Gabella isn't a Mary Sue, nor is she omnipotent or unbeatable; Nemad's still in chains and bound, and so can she. The fact that she and her armies have, you know, been defeated (and are regularly defeated in-game) sort of proves that she can't do anything she wants. Even when she 'needs to', she either doesn't or can't bring forth the powers to win. And since she's arguably one of the most motivated characters in the lore, 'doesn't' isn't a good excuse.
I really don't see the justification for such fan-worship for her. Even without killing, there are many ways, especially in fantasy context, to nullify/neutralize/incapacitate her. Just like Nemad.
That fact that she did do it the one time she was defeated should be enough to show what she is most likely to do for this hypothetical question.
She (Faeryl) replaced her sister, a fellow elf, someone with whom she had extensive experience and personal knowledge of in a culture that was not so differerent from her own (not least because they were the same).
Faeryl really doesn't have claim to be able to fool, say, Luchirp by copying Beeri Beeri, or the Ilians and Auric, or Basium, or pretty much every leader
but the one she personally knows in and out.
I really don't see how you can assume the Elven Civil War is anything but a special case, even for lore. Faeryl makes a poor Amurite Archmage/Lanun playboy/Selfless pacifist/Hippus warrior/Luchirp/insane clown/angel fanatic/demon fanatic/ midget dwarf man. IE, she makes a poor pretty much every other civ male, and most of the females.
The point is, saying "They wouldn't be able to do that because I would stop them" doesn't help advance this thread topic much.
Except, according to the thread, you already have. This isn't a question of if you can beat them and their civ; you already have. So yes, you can stop them. The fact that this question is posed already proves/assumes that.