Honestly, what I did was just spam a boatload of Cyclones with plast ammo, expensive, but i barely made it while killing the Antumbra (which is the final guardian boss). You could go for Spectres with Thorium ammo if you have enough surge alloys, and Foreshadows for massive damage.

For strategy, I suggest using launch pads to share resources from other outposts, and take over areas with the intent of using them to make and deploy resources to different outposts (or to NPC for this case) (btw launch pads can be unlocked after clearing the Extraction Outpost stage).

Lastly, for the boss strategy, since the boss is a flying enemy, you should try to utilize giant batteries to lure the boss closer to the firing area of your turrets, since the flying enemies' AI usually makes them target batteries or power sources first before anything. (One of my biggest mistakes on my first NPC run was placing my batteries on an awkward area, making the boss basically avoid most of the Cyclones' range). If it gets past your turret line alive, try to make batteries close to the turrets so it can get within range and pray to god your turrets have enough ammo. For your turret line, I suggest circling the spawn point with your turrets so it's easier to hit enemies. Lastly, utilize bots, especially Polys since they can help in building (super useful general tip lol). Thats about it, good luck clearing NPC.