I'm assuming that the mission will be much more discreet then simply just "Kill this person". Otherwise, we would all be able to solo the mission.

And Necro, even if Nova causes Zero to "think" he lost control of his geass so long as it didn't happen the timeline's still sunk, as the fact he lost control of it is still important. It's a demonstration of it's growth that will eventually lead to the geass emerging in the second eye.