How Little We Know - Killing Big Sal and Nero
It seems that Big Sal figured out what was going on. Cachino was put in charge of the investigation, but he can't really protect you. We're down to the wire on this one. You and Cachino are going to have to take out Big Sal and Nero when you report to their office.
If you can, hold off until you get your speech above 80. Remember that you can always get a few drinks to boost your Charisma a bit.
You will probably want to have Cachino hold off and wait for your signal. Hopefully you got a few real weapons from the cache. If not, you'll be stuck with either your holdout weapons or the sawed-off shotgun that Cachino will give you before you step inside. If you have to use the sawed-off, remember to bring slugs for it. This should make sure that it's actually useful against Big Sal and Nero.
Note that if you're really having trouble with this, try using Veronica. Her power fist is considered a holdout weapon, so she can sneak it in for the figh t and probably take out Big Sal and Nero in a few hits. The same should also be true for Rex, naturally. Your other companions will probably be of limited use though.
Once the conversation starts, just listen for a bit. This is your one chance to learn the real plot, so just let him keep talking if you're curious. It seems like you ruined a massive war plan and wrecked Caesar's plans too...so this isn't going to end. You can attack at any point, but there is a unique option. If you have more than 80 points in Speech, you can convince Big Sal that Nero is the real traitor and that you were just his agent. Just use the option saying that “Nero didn't tell me that part of the plan."
If you used the speech check, Big Sal will first attack Nero and kill him for you (although it's AI combat so the outcome is anyone's guess, apparently they can even manage to one-shot kill each other, although that's unlikely). Cachino is pretty fragile, so try to get into VATS quickly. You probably want to keep him alive to make sure that the quest finishes properly.
The sawed-off shotgun isn't bad at this range, but it doesn't pack much of a punch if you don't have slugs for it. Use the heaviest weapon that you have and blow Big Sal away before he kills you or Cachino.
Talk to Cachino to receive the paltry sum of 50 Gomorrah chips for leading a coup and appointing him as the new leader of Gomorrah. He is loyal to you though, so that's something. Report your success back to whoever you were working for to officially wrap up the quest.