From GameWatcher: “The Undying King is a tricky beast. Not that Remnant: From the Ashes isn’t chock-full of difficult bosses, but we can’t help but think that the Undying King boss fight is one of the hardest encounters all around. This guide will try to help you out with that.
Of course, this guide presumes that you’re not too keen on working for the Undying King, because Remnant gives you that option, too, in classic Weird Fiction fashion of subverting expectations. If you’re dead-set on killing this boss, keep reading.”
Source: N4G PC Remnant: From The Ashes – Undying King Boss Fight Guide