NFD built a "FDS" (Fighter Denial System)
Ultimate swarm missile in an extremely compact package. Could be the perfect weapon for this if and I say IF it actually deals any damage
I think I covered it before, the Swarm missiles are theoretically the best weapon against drones, but they have the flaw that every missile targets for itself, so they might all crash into one drone, they might crash into them at an unfortunate angle (not killing it outright) and they have kind of a high reload time.
Edit: Took 2 of my anti-fighter turrets, replaced the slave cannon with missiles and pitted them against a few Isanths. Turrets heavily damaged, the Isanths shot to all hell, but defeated.
The spread pattern makes it difficult to hit the core, even with punch-through and the fire rate is too low. Also (if you care about it, and with a shipyard system, we might) you will not have a lot of salvage from such a battle, most Isanths had whole rows of valuable blocks missing.
Stick with cannon/cannon/punch-through, it might be your best bet. If someone would remake cannon/missiles to be a multi-shot flak cannon, that would be better than shotguns, by far.