Ok, so other than the fact that Arlong scales below the amount required to do any damage to Soldier Boy(remember, Homelander took essentially no damage from that chemical plant explosion), he's got skin that's resistant to piercing and slashing. Plus, he has his shield, which has even better durability than he does. Even if Arlong was just popping out fancy-ass Fishman Karate moves(he's really not especially good at that), Soldier Boy's insides are equally durable. He's got a gun and a knife on his profile too if he loses his shield. He can also shoot out huge AOE radiation blast for tens of meters that can just eradicate people and reduce buildings to charred ruins. And to top it all off? Lil' bit of regen to help him keep going.
Arlong gets stronger underwater, but certainly not enough to overpower Soldier Boy. He gets faster, but that's not enough to net the win. He can throw water really fast, akin to bullets, but his foe is bulletproof. He can also enter a berserker rage, but he's still inferior. His LS is superior, but grapples are dangerous against a guy that can, as mentioned before, do a crazy AOE blast while emitting significant heat merely by charging. His nose is durable, and his sword is sharp, but neither is infallible, and he's between a rock and a hard place here, I fail to see what he can do.