La Dama Negra
As for most of the Legendary ships, attacking from behind is the way to go. For this ship however, it’s the only way to go. This heavily armored ship is unlocked when you take the Seranilla fort located to the southwest of the map. Once it is roaming the sea, approach it and your crew will comment on its armor siding being impenetrable. This ship also has one other sneaky attack going for it; it has mortars that it constantly fires, except there is no targeting on the water to show where they will land.
To damage the ship you will need to get up behind it and constantly barrage it from there. It may take a bit of time as the mortars will be able to hit you back there, so you’ll need to keep moving and not slowing very much at all during the fight. Take your time with this ship and retreat when necessary to a distance to gather yourself for another attack on it. A good maneuver to do is to get up behind the Negra and chain-shot it to slow it, mortar it, and then turn and nail it with a heavy shot or two directly on its rear cabin.
This ship is one of the tougher Legendary ships that you’ll come across and the weather and choppy seas don’t make the fight any easier, but knowing where the weak spot on the ship is makes the battle much more bearable. Just keep on the butt-end of the ship, give it a couple hits and get out of dodge to prepare for the follow up and repeated attacks and it will sink soon enough.