As I said earlier in the thread - search on YouTube, I promise you for every final boss in the series, you'll find someone insane/good enough to fight and defeat them with a single character. :p
gameplay/etc
Cop out answer, man. You can't just dismiss a complaint about the setting/consistency as WHO REALLY CARES??? I do, at least enough to call them out on it.
Second, why is it beyond the scope of believability that the heroes would also be in turn, stronger as well? Did that ever cross your mine as a possibility as to why they're able to now fight with their villains one-on-one as well?
Well, why are the heroes suddenly so powerful enough to fight their foes one on one? Was that explained in Dissidia? If it was, that's fair.
It's the same reason Cloud was able to hold his own against Sephiroth in ACC in the first place, and you were more than able to accept and argue the fact of his sword prowess within that department too.
Like I said, I though, and think, AC(C) was and is stupid. However it's not completely unsalvagable, and Cloud and Sephiroth fighting one on one is more believable because they're
both stronger and time has elapsed between their last battle. The other FF heroes/villains have no such thing. Terra and Kefka, or Squall and Ultimecia didn't have any timeskip where Squall or Terra could 'catch up'. They're both fighting as is.