There are four total legendary battles in the North Atlantic to gain this achievement. Anyone familiar with Black Flag has a good idea of how this works, though the specifics have changed slightly. In Rogue, only 3 legendary battles are available from the beginning and you must complete them to unlock the fourth. These three leading battles are a trifle compared to the annoyance of the Black Flag legendaries and you should be able to smash them with few upgrades to the Morrigan. The final battle against a ship called Storm Fortress which is a huge pain in the ass because it's a Man o' War that is faster and more maneuverable than your ship.
You're going to need a mostly upgraded ship to have a snowball's chance in hell of winning this fight. You absolutely need elite hull armor, the blueprint for which is found in Genessee in River Valley. Just about everything else should at least be upgraded to the point where the only remaining upgrade is unlocked by blueprint. On that note, though, none of the ships in this battle expose weak points so the puckle gun is inconsequential and I never used the burning oil, so these two you can pass on without making your life any more difficult.
As for the fight itself, winning is all about the golden rule of the Man o' War: it's defenseless and weak from behind. When you spawn into this fight, you are the flagship of a small fleet that gets decimated within 45 seconds and there's good chance you'll be responsible for killing most of them. I ended up ramming two or three of my own guys because they were in my damned way. At any rate, Storm Fortress handles exceptionally well for an enormous tub and is very skilled at keeping you within the reach of its devastating broadside cannons. This is where the ram comes in. No, you're not going to ram the ship, you're going to outpace it.
From the get-go, look down the bow of your ship and use the ram (
) for a speed burst to break away from your fleet and get behind Storm Fortress. The one speed burst from using the ram ability should either get you very close to where you need to be or right in perfect position to start hammering this pain in the ass. The fight you're going to have will be in two distinct phases that alternate. Phase 1 is unloading broadside cannonballs into its' tender ass, and keeping pace with it as it spins donuts fruitlessly trying to get you in its broadside. If you can use heavy shot then go for it (just tap
when looking out to the side to use heavy shot), but I found that aiming with
to use normal round shot prevents you from getting into trouble too quickly. Phase 2 is when Storm Fortress decides to run away and mortar you, and you want to use Morrigan's ram ability to keep pace. Go through the flames if you have to because attempting to go around will either put you in the ship's broadsides or find you much too far from your target and getting pummeled with more mortars. Storm Fortress will always slow and start turning again after it buggers off, so you simply want to catch up and get behind it again. Using the chase cannons on the front of the ship (the carronades) will help immensely because they slow and inhibit movement on impact, and the shot usually makes Storm Fortress stop running.
Just keep repeating the pattern until dead. Unfortunately, when Storm Fortress gets to about 25% health then two more Man o' Wars will turn up. They fire all kinds of heavy shot as well but these two ships at least have the decency to pause and reload their cannons between volleys, so Storm Fortress is still your primary target. Once the big bastard sinks, the two remaining ships follow a predictable pattern of coming in for a broadside and running away again, so pepper them with mortars as they run away and unload whatever you've got when they get in close. Killing one will send the other into suicide ramming mode, so do your best to evenly distribute the pain before finishing one off.
Your biggest problem is the bad boy you're there to kill by a long shot. I only lost one bar fighting Storm Fortress and one bar killing the other two ships, so you should be in good shape if you can limit your damages similarly.