Use a decent shotgun w around 600 ammo and a cactus broth tgen you can do the boss solo

Take a few good pumps and cactussoups and shoot at his arms! Dont get to near to the nameless and Reapers!

Well easy on foot with 200+ damage shotty around 600 ammo and a cactus broth enabled and just shoot it done

The artifact in the radiation zone is not that hard to get, there is a reason ark supplied us with a rock drake, and the reaper king send that sucker off to kill whatever he pleases while you go get your artifact