Your best bet is to go for this one in a Co-op vs AI match on Casual (both Public and Private) and try to get either Harbor or Harbor Haze. The mode doesn't matter, but obviously modes that last longer like KOTH are preferable. While a Private match will be much quicker (potentially) since you can select the map, you can only do so if you own it. So if you don't have the season pass and you can't see it in the menu, you'll need at least one friend who does to be in the lobby.
Get to the where the ship is and set up around the horn. The horn is that lever which is situated where the Embar and Longshot spawn. Take turns pulling the lever by pressing
when standing in front of it, while trying to keep the enemies at bay. The horn takes some time before resetting and allowing the next person to pull it, so keep an eye out for when it resets and get the next person to pull it.
It doesn't matter if anybody dies or if the round ends. The achievement will pop as soon as the fifth player pulls the horn before the end of the match.