League-of-legends – What’s the best tactic for a fed carry (AP or AD) with normal offtanks against a fed offtanks with normal (AP or AD) carry

Sadly, there's no sure-fire way of resolving this situation.

You won't be able to rely on minions to take out their towers and nexus, especially if their uber status gets to the point where they can start defending their respawning inhibitors. If you get really lucky and get their nexus towers down, you could just farm minions hard enough for yours to take the nexus down a bit then try to dive the nexus. I'd consider this a Hail Mary more than anything else, though.

If you know you're going up against a team that is going to steamroll you once they get maxxed out (2+ carries), the main goal is to take advantage of the fact that they will be weaker in the early game. If you zone the carries and/or kill them enough to keep them from getting geared and leveled, your team will get powerful enough to take their nexus before they can do anything about it.

Assuming you find yourself in that unfortunate position anyway and forcing their team to split up isn't working, you absolutely have to focus the primary damage dealers down first. If they stick together and you're not killing their carries first, you'll take way more damage than you will deal out over the course of the fight and you're sure to lose.

Nocturne to me is an "anti-carry". Make sure to play him in a team that can handle you not being around them in a teamfight. After you dispatch their squishy carry, fear the most disruptive member of their team to minimize their impact and then focus on the next highest priority target.