Option 1: Skip it.
Option 2: Kill the command station, hopefully triggering a mass attack on a neighboring system that doesn't have an AI eye. Then send in some stuff to run around and snipe the remaining guard posts before the zombies overwhelm them. You'll have to deal with the zombies eventually but you'll have the rest of your fleet for that.
Option 3: (possibly multiple waves of) 250 bombers
Option 4: Some micro-heavy thing I've never done involving aggroing the guards near that guard post and running the hell away.
Option 5: A superweapon you don't have. Golems or fallen spire stuff.
Option 6: EMPs may disable the force field guardian, allowing you to snipe the arachnid guard post (assuming that's what you meant with "starship devourer") with bombers. Be warned, however, emps do not disable fortresses or ai eyes.
Option 7: Cutting supply to the planet (killing the ai command stations on that planet and all neighboring ai planets) will disable the fortress.
Option 8: Nuke.
Alternatively, if you meant spire field guard post and starship disassembler guardian.... uh, skip the system, nuke it, or make lots of bombers. There are some weird conditions that can get force fields to self-destruct though. Something like "if there's nothing under them and the command station is dead, they pop". But I'm not sure exactly what those conditions are, so perhaps someone knows better.
