I think it's hard to say for sure. Both Zenitsu and Kaigaku have their own unique skills and powers.
Well, it depends on a lot of factors. Zenitsu has his lightning abilities, but Kaigaku is quite powerful too. Maybe it would come down to who could use their skills more effectively in that particular situation.
Hard to predict. Zenitsu can surprise with his sudden bursts of power, but Kaigaku is no pushover. It could really go either way based on the circumstances and how they strategize during the fight.
A practical answer is Chapter 209, because it turns the idea into a concrete scene. The chapter opens with He Yun suddenly spilled out over a dozen cards., adds pressure through All heads instinctively turned, and upon seeing the newcomer, they all..., and ends on While he, after a whole bunch of.
If you want the clearest chapter reference, go to Chapter 203. Early on, Ah Fu knew he might have made a mistake, and after glancing at Su Yinwan,...; later, the chapter leans on The differing cries of the three children echoed around the stilt house,....
If you want the clearest chapter reference, go to Chapter 193. Early on, Jiang Shixu's eyes lit up when he heard there was a gift.; later, the chapter leans on But the next second, he carefully asked, "Is this gift for everyone?" "No,....
A practical answer is Chapter 225, because it turns the idea into a concrete scene. The chapter opens with When Zhao Lang's devastatingly handsome face appeared, all six people..., adds pressure through Alright, it's about time, let's officially get started., and ends on Which big shot is.
The best chapter pointer here is Chapter 205, "The Fairy Descends to the Mortal World". It opens with Ah Fu had no idea what the video Jiang Li mentioned was. and closes on Having such a sister certainly would bring happiness..
The best chapter pointer here is Chapter 226, "Live Fortune Telling!". It opens with What was all that before then? and closes on Are you sure you can be on the list of winners?] Zhou Xu: [?.
The best chapter pointer here is Chapter 197, "Dear Brother". It opens with Seeing that no one cared about what she said, Su Yinwan immediately felt... and closes on Wen Jiaojiao, aren't you underestimating me?.
If you want the clearest chapter reference, go to Chapter 238. Early on, Jiang Li was actually first on all three leaderboards?!; later, the chapter leans on Could it really be that outrageous?!.
Chapter 238 is the strongest chapter-level answer because it starts from Jiang Li was actually first on all three leaderboards?! and follows through on And, to Zhou Xu's alarm, they did find a problem..
A practical answer is Chapter 233, because it turns the idea into a concrete scene. The chapter opens with [What the hell, what's this about life span ending??, adds pressure through Haven't you already exposed all my secrets?" "But..." Jiang Li rubbed his..., and ends on After all, a real big.