Darth Nox, because she can't die. Remember? Thanaton tried his hardest on Dromund Kaas. A Darth Nox who still has the ghosts after the endgame can't be killed. This is defined by the story. And given that the only person who could teach the rituals of Ergast is Ergast himself (who's trapped in Nox) and with the rituals of Tulak Hord being in Khem-Zash, it's doubtful the Wrath would have any idea about how to get rid of said ghosts.
That being said, I find the whole concept of this fight interesting, though unnecessary. In terms of the story we know the new Emperor's Wrath is in fact not made aware of the Emperor's true plans...will be interesting to see what BioWare does with the story there. If the Wrath knew the Emperor's plan, they should also know that Nox would be one of the few Sith with enough knowledge of the ancient rituals, and the power to help do something about it.
This has given me a rather hilarious image...
The Wrath repeatedly stabbing and hacking Nox who keeps trying to explain the Emperor's plans and why they're the only one who can stop him. While the Wrath just keeps shouting 'WHY WON'T YOU DIE?!'