In Assassins Creed IV: Black Flag I'm not sure which strategy to use for big ships, I usually go with a massive Mortar attack, but its not that good.
Here is a guide for all the legendary ships:
HMS Prince (Navassa Zone)
This ship is similar to the Man'O War ship but with heavier hits and defense. It will try to get away from the Jackdaw with its superior speed while using Mortar on you. Similar to other mortar attacks, you will see the area of effect, giving you a small opportunity to avoid damage.
Try to move close and use the chain and ram strategy, trying to stay as close as possible so that it prevents the HMS prince from using Mortar. The HMS Prince is probably the easiest of the Legendaries.
La Dama Negra (Serranilla Zone)
The La Dama Negra is very similar to the HMS Prince in terms of its attack style with one key difference being that unlike other mortar attacks, it does not show the area of effect.
One way to respond to the mortar attacks is try not to move towards it directly from a distance, as it will most likely fire devastating mortar shots in your path. If you are circling the La Dama, slow down a little when you see mortars coming, as most of its attacks are adjusted for where you are speeding towards.
The best strategy against this ship is to stay close and use heavy cannons while ramming where you can, staying close will also mean reduced chance of it using mortar on the Jackdaw. Remember that her sides are heavily armoured. Moving behind her will allow you to do normal damage.
HMS Fearless And Royal Sovereign (Eleuthera Zone)
This legendary battle is a fight against two with less HP than the rest of the Legendaries, they will come at you by funnelling you between them and using their side cannons against you.
Best strategy is to chain shoot one and then ram them, once the boat is pinned down continue to use chain shot and ram, making sure the boat doesn't get too far away from you. Another crucial part of the battle is to use brace when your boat is between the two.
If you destroy one boat before the other, the remaining will employ a different strategy where it will constantly ram you for massive damage, so another strategy is once you've damaged one ship's health to about 10%, start using the chain and ram strategy on the other ship so that once you destroy one ship, you can finish the other one with one or two mortar shots.
El Impoluto (Dry Tortuga)
In terms of difficulty the El Impoluto is probably the hardest of the lot as it is a lot more aggressive and has massive ramming power compared to the other Legendaries.
Strategy against it is to shoot chain and then ram where you can, however, given its superior movability compared to the Jackdaw, you will also have to employ cannons and mortars when El Impoluto runs away from you, it will respond with cannons, so remember to use brace when you see cannons coming.
Another thing to watch out for is when it moves behind you, the best defence to deter it from ramming you is to let off a few fire barrels and let off a mortar to steer it away.
For el impoluto the best strategy is when he is comming right at you just aim for the space between the ram and the side and if you have elite ram it will do a ton of damage as for la dama negra 3 important things, 1: dodge mortars.2: never aim at the sides or the front. 3: always aim for the back and for the hms prince, stay on his tail like you're glued to it and once there, do whatever you want.i never beaten the hms fearless and the royal sovereign.
Here's a glitch to beat those Legendary Ships EASILY:
Description of glitch: Accelerate at travel speed towards the Legendary ship, then at 570 meters disengage from the wheel. Shortly after, because Jackdaw still has some speed, it will float into the range of the Legendary ship and the cinematic will kick in.
The combat however, will not start as Edward is not holding the steering wheel, but legendary ship will continue to advance towards the Jackdaw. At that point it is possible to jump over to the Legendary ship from the bow of the Jackdaw and assassinate all of the ship's crew, which renders legendary dead in the water. One volley of cannon fire will destroy it.
I beat the el impoluto easily by basically beating at its own game I just steamed towards it at full sail and as soon as I was close enough I used a fully upgraded chain shot and rimmed into its side then veer off to a side and use broadside and swivels and repeat those steps also don't be afraid To ram into it
You can easily defeat El impoluto ship by boarding her in this strategies: A 1.) Mark her in map or by using scope. 2.) Dont use speed travel just use two sails. 3.) Wait until you get closer in her in 580-589 m and hold STop to release wheel. 4.) Quickly run Edward to Jackdaw's bowsprit. When she get closer to you, LEAP Edward to board him to ship, 5.) Kill all Crews, Captain and Scouts and go back to jackdaw to fire cannon to sink her.
Remember :you must walk only on deck, dont climb on boats and use stairs.
If you fail to do that, and youre not board in ship, and Edward is swimming around the ship but El impoluto is Stopped, closed sail and anchored.
Just do this B 1.) Go back to jackdaw 2.) See if Jackdaw and El impoluto's Sail holder is meet to each other. 3.) Climb mast or ratlines, walk on sail holder to el impoluto's sail holder. Dont leap or jump. 4.) Kill the scouts, use ratlines or mast to down to deck then kill all crew and return to jackdaw. Thank you, sorry for my grammars.. kenn.ysrael.22
To take down a standard Man o' war (Galleon) is quite easy once you have + Tier 3 or 4 upgrades and some practice, from personal experience, you can use the "Broadside cannon and swivel gun strategy" this tactic is relatively quite simple.
Fire with your broadsides at the ship, mainly the bow or the rear.
When the crosshairs come up hold Triangle (On PS Systems) until the other crosshair matches the other and let go (If you have elite just hold it)
Now just circle the ship while repeating the 2 steps above. (Don't forget to brace when the Galleon is firing because if you don't have good hull upgrades, the broadsides do TONS of damage)