Since you have the XP cap removed, try the spell Level Drain. I was fooling in the tutorial for a while and wanted to see if I could kill Belt, who I thought was invincible as well. I lowered his resistances and just kept level-draining him.
I dunno if this works against Arkanis or William tho.
Another guy who is immortal is Rejiek Hidesman, the murderer in the Bridge District. I cheated and made an all-paladin party, all equipped with Carsomyr and boots of speed. After finding all the evidence, I went down to pin him. I was able to corner him in the second floor of his house, started beating on him like a rented mule. Strange enough, he suddenly vanished and reappeared, and he was made immortal!