I would presume that it would be impossible for any Apostle or God's Hand (or even all of them) to oppose Idea, both since they are still within fate, with Idea being the one who controls fate, and also because Idea is the source of their powers. I doubt they can use those powers against Idea, who gave them said powers. Besides, why would they want to?EDIT: ERROR, that's not it. can griffith and the god hand gather enough power to oppose idea? Can all their powers, skull knight, Caska, Griffith, Guts, can their combined power slay idea? Or is that another error in my logic.
As for any one else killing it... hard to say but I doubt it. First, how would they get to it? It exists deep in the Vortex, essentially impossible to reach, unless it brought you to there (like Griffith). Second, how would you kill it? Idea was born from the collective unconsciousness of all of humanity. How can you kill something like that? And even if they somehow did, what would stop it forming again? After all, even if you somehow killed it, humanity would probably just create it again.
altough I think that could be sheirke's job.
