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!

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