There is a little pointing rock formation (looks like a claw/tooth sticking out of the ground - RED CIRCLE) just opposite the stairs from jokos prison. Get on top of that and spam ranged attacks. Don't use any boon giving abilities, in fact only use your "1" skill/auto attack until he dies. His jump attack AOE will hit you for little damage and his soul leech will not pull you off. Just make some coffee and wait till he's dead (but don't die yourself).