I'd wager she's more than enough to hold off an unamped Nyriss if she can hold off an Orbalisk Bane before her prime (whose formed Whirlwinds with his lightsaber, blitzed Sith Assassins, and made his blade appear everywhere), I also don't see the Exile penetrating her defense at any point unless she uses the force.
Malgus can defeat either and aid Zannah in closing the victory. Her Soresu is perfect for Darth Nyriss, and all she has to do is hold off the Exile
The Exile is not susceptible to mental attacks and I highly doubt Nyriss is. there is no DS Nexus for Zannah to use sorcery with in this battle. All she has is defensive lightsaber play and both Surik and Nyriss are masters of forms that work around that. Her contribution will be little in this fight besides buying a short amount of time.
Darth Malgus is the problem here but he can't take both of them by himself.
She was fighting Nathema's attempts to rip her apart and absorb her Force energy, she was there far far longer than a few minutes and Nathema's effects are far stronger than Darth Zannah's mental attacks. Furthermore she resisted the power of Malachor V and Nihilus' domination powers before these events.
On the other hand she was taught to guard her mind by an expert in the area Atton Rand and was capable of achieving telepathic prowess that would take decades for average Jedi Masters to learn. Perhaps most importantly is that she was taught by Revan how to defend herself from Vitiate's mental domination, Vitiate's TP is much more powerful than Zannah's by a large margin.