Cartrimino I don't think it should be nerfed. I've done the Black Sails challenge and have completed it quite a few times. So it is possible to do is my point. Any easier and I don't think it'll be as rewarding or satisfying to finally do. It's also one of the rarest achievements in the game.
I'll share my method for doing it. What I've done is you have one person knocking them out as soon as they jump over, before they even stand up. A running punch while they're still laying down from jumping across still works on them. Just practice knocking them out as soon as they land. Meanwhile you have the other guy distracting and collecting all the other ones who actually made it across and have them chase that person around the ship while the other person is still spawn camping all of them jumping across. As soon as they stop jumping across then the person who was throwing them off and spawn camping them should go over and help the other guy who has some enemies left over following him and help knock him out and throw them off. Bringing a weapon to help is very useful but can be risky. If the one being chased has no enemies then helping to spawn camp them is best - especially on the last wave when they just keep coming. Usually the method begins to fail because of the rapid deployment of the enemies. However maintaining this method can clear out a LOT before you get too swarmed. Cutting it down from 8 on your ship to maybe 3.
A couple things.
For the person being chased - the enemies tend to jump kick sometimes so usually it's best just have them follow near the edge and watch them jump kick themselves right off the ship. If the person being chased starts focusing on fighting instead of keeping them busy then it gets very risky. It's essentially no longer the method I'm explaining and basically just fighting apart from one another. Something you never want to do since saving each other is crucial. Stay close together. If the one being chased gets knocked out, it can make the one camping to stop to save them - allowing more enemies to jump across and swarm you. Not getting knocked out as the one being chased should be the highest priority.
Another thing. If during the last wave you get absolutely swarmed with like 8 enemies because the person camping couldn't keep up (its okay, its hard to do) then, as long as your ships health is high, use the timed bomb shot onto your ship to blow them up and help clear them out.
Some say online that there's a couple spots where one person is untouchable or is safe. On the Bannister near the cannon apparently. I haven't tried it but maybe this could be a much more safe way to get them to "chase" that player while the other spawn camps.
Take out the crossbow enemies as early as possible. I haven't tried it but maybe you can shoot enemies before they even jump over if you steal the crossbow. Or you could use the crossbow as the person being chased to shoot enemies straying off to attack the camper to get their attention back to you. But regardless if you use it or not, they're effective againts you and need to be taken out ASAP.
If you time it right, if you throw an enemy off the ship where they jump across onto your ship - you can throw out the knocked out enemy and obstruct the one jumping across. Effectively killing 2 of them.
The first 2 to jump across on this level don't jump kick. So just camping them climbing up is easiest. The next two that come across, only the first one jump kicks across. So repeat for the 4th enemy jumping over
The enemies target based on proximity, visibility and who attacks first. So this means when they jump across and the spawn camper misses, the chasee must aggravate the surviving enemies to follow them instead of the spawn camper. If the camper is being swarmed. Just knock out a few enemies to shift their attention to you, then get them to chase you away from your teammate to relieve them.
Staying still supposedly "de-aggrovates" them. So the person being chased should be the one to move the most to hopefully attract them instead of the camper who should be moving less.
Supposedly being in higher elevation makes you more visible. So the Bannister or the cannon deck in the back are prime spots to distract enemies and its hard for them to hit you. Swinging your arms and moving around will keep their attention on you.
This is what came to mind. I'll send more tips if any else come to thought.
Good luck!