Magicarp are fast way too fast and in a state of panic everyone will not think rationally
Quote the OP:
15,000 random RL humans each with a random weapon (9-A tier or lower) vs 10,000 random Magikarp, some below 9-A, some at 9-A, some at 10-C, and some above 10-C. Half start in water half start on land. The two sides start 300 feet away from eachother. Battle takes place on a very large beach.
Also:
The toe stubbing statement would probably only be true for guns of 9-C and lower anything higher than that would do a great deal more damage than toe stubbing. Also as their are 6 tiers in this fight (10-C,10-B, 10-A, 9-C, 9-B and 9-A) we will assume there are equal numbers (1/6) of each.
Only about a third of the Magikarp are 9-A or 9-B.
Or about 3,333 or so.
Plus, the humans outnumber the Magikarp by 5,000.
Even assuming Magikarp's rating applies to its travel speed (The idea of Transonic flopping around is amusing, at least.), Magikarp aren't that intelligent, & may not be that coordinated.
& giant fish that the humans might think are airdrowning might be a source of confusion more than panic. Regardless, they only need a few humans to be rational to succeed, & the Magikarp, even if they are hostile, likely don't have the sense to identify the least panicked humans, let alone target them from the start; How would the Magikarp PREDICT plans like that?!
Even if these Magikarp were that prepared, the numbers aren't in their favour, since they won't know WHICH humans have a good idea or not, & slightly less than half the Magikarp even ARE strong enough to one shot humans.
Not to mention, the usual means for Magikarp to get into the human killing tier is for the yield of its falls:
"A MAGIKARP living for many years can leap a mountain using Splash. The move remains useless, though."
The jumps that lead to those falls aren't common, & would take valuable time anyway. Almost all of Magikarp's other moves are from rather rare means, so they may not even be IC.
"An underpowered, pathetic POKéMON. It may jump high on rare occasions, but never more than seven feet."
Most of the Magikarp probably won't be fit enough to be a threat, & it may not even be IC for them to behave so.