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What is worse than a single level 75 Man o' War with massive armour and overpowered weapons? Why, two of them at once of course. This legendary ship encounter is by far the hardest in the game. Not only are both of these ships more powerful than anything else you have encountered in the game (except other legendary ships) on their own, you have to fight both of them together and they work as a team.
The most important part of this fight is to stay as far away from the ships as you can, dropping mortars on them from the maximum range possible and making sure that you never, ever let them flank you. They have an extremely powerful broadside attack and if both of them unleash it on you simultaneously you are in serious trouble.
With fully upgraded mortars and broadside and full ammo, I was able to stay at a distance and rain mortars down on whichever vessel was in range at the time. By the time I was out of ammo, a few convenient passes with heavy shots and swivel gun action on some weak spots saw the first vessel sink. Afterwards it was just a matter of focusing on the second ship and whittling its health away too.
Do note that when you take out one ship, the other will set itself on fire and gain a huge burst of speed and a ram that decimates even the sturdiest hull. You will want to make sure that the final ship is significantly damaged before it does this, otherwise you will find yourself replaying this battle repeatedly.