I feel like Don Krieg has the edge here because he has poison gas and powerful weapons. Plus, Arlong can't use water to his advantage because there is no nearby water. Moreover, when Don Krieg was shooting bullets at Luffy in the Baratie Arc, Luffy didn't deflect them and I'm assuming it's because Luffy simply couldn't because why would he intentionally not deflect the bullets? I also think the bullets would also be able to pierce through Arlong's skin aswell.
