From your description, without looking at the save it sounds like you're talking about a main hub on a spokes map?
That can be tricky, but I still think that a station raid would be the way to go here, especially because of the core forcefield. Even with a forcefield up, you're never in a position where a raid is fully out of the question: it's just harder. What you want is something that can get through the forcefield to kill it, like Raid Starships for instance. There are a variety of ways to go about creating diversions, etc, for your Raid Starships to sneak past (even an EMP missile could be the perfect ticket there -- probably what I'd do), and then it's just a matter of getting them to the station to kill it before they die.
After that... ships swarm through your wormhole and hopefully die on your waiting fleet and turrets, and then you're left to pick on the turrets, forcefields, and whatever else is left.
Others might have more specific ideas by looking at your savegame, but that's my general advice for that sort of situation.