Hisoka is not much stronger than Kunckle. He's not even necessarily physically stronger than Gon by the end of the Chimera Ants arc.
But the problem with Knuckle's abilty is that it almost surely only works on basic damage. That is say he's up by 10000 and someone pulls out an Excalibur and try to chop his head off. I don't see how his ability will somehow prevent this from happening. The damge is coming from the weapon, not from the user, and we know there are weapons powerful enough to penetrate the defense of an aura user due to their own strength. Likewise if someone pulls out a flamethrower and set him on fire, that too should work just fine.
So while Knuckle has an incredibly strong ability for a duel, in a fight to the death all you have to do is just use anything that does special damage (slashing/fire/electricity/poison/whatever). The fact that I don't see how Potclean can grant slashing weapon immunity makes it rather easy to deal with. If Potclean does affect slashing damage, that'd mean you'd get a scene where someone as strong as him uses a sword and try to chop his head off only to have it bounce back due to the amount he owes. That just seems ridiculous so I assume it's not possible.