A good rebel fleet of Han's Falcon, Biggs, Bistan in front with Ghost, Phantom, Cassian in reserve can easily handle a Negotiator. But does require a significant investment in the front line pilots. And generally most of them are not much use elsewhere.
Now as shown in the link provided by Antario, Geos can beat Negotiator. I believe this has also been done using Chimera. I recall a thread on this before. But whatever route you take it is not just about the fleet mix. It is about the crew gear and technique.