Spirit sword won't be able to one shot knuckles due to Hakoware, he's actually the perfect counter for Kuwabara.
Since it will just return the aura to him.
While Spirit sword is extremely taxing for Kuwabara himself, Knuckle could just wear him out/bust him since it's his usually strategy against strong opponents.
I'd say knuckle take both round 1 and round 2, maybe round 2 with more difficulty since Kuwabara is a little faster.
Round 3 goes to Kuwabara and round 4 is a tie.