Assassin's Creed Rogue - Master of the North Atlantic Achievement (Trophy)
There are four legendary battles. They are located on each corner of the North Atlantic. Beat the first 3, to unlock the fourth, which is located in the bottom left corner. It is highly recommended to fully upgrade the Morrigan before attempting these.
But if you're just trying to get this achievement, I have found that using the Immortal cheat doesn't disqualify you! Here's what I did:
1. Beat the first 3 battle legit. They're not too difficult.
2. Fast travel to the bottom left corner of the North Atlantic.
3. Turn on the Immortal cheat.
4. Sail to the last Legendary battle.
5. After the battle ends and you're back in the sea, exit to the main menu.
6. As the main menu loads, the achievement popped.
00:00 1st: Legendary Battle (Pirates of a Lost Age)
03:52 2nd: Legendary Battle (The battle of Labrador)
10:30 3rd: Legendary Battle (The battle of Quiberon Bay)
15:16 4th: Epic Legendary Battle (Pirates of a Lost Age)
15:25 before Battle start : Turn on the Immortal cheat (optional: Corucopia Cheat for Unlimited Ammo)
21:38 Turn off Cheat Mode
21:59 Master of the North Atlantic Achievement
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