Oddly enough, the NE, SE, SW, ships took me about two tries each in a sitting. It was only El Impoluto that gave me the hardest battle...3 times with just spamming mortars, fire barrels, which is the only time I've actually used them, and trying to stay on it's ass the entire time. The battle sucks because it takes place in a storm so you can't go full sail, where the Impoluto, can despite being a larger ship. Wonderful logic from the developers point of view.
It took about 5 for each. 3 times trying normal, then 1 with infinite health for your ship and infinite ammo for your ship cheats on for the heck of it for good practice. Then went and took them down for real. The last one was El Impoluto. Which I put on the cheats for and got the achievement for defeating all legendary ships. The achievement didn't pop up until after I took down and boarded another ship. Then after, I turned the cheats off and the achievement popped up.
Weird, yet I still would like to take on El Impoluto for real at some point. Which is still shown on my map if I have the urge to do so.
Waited until the end of the game so I had an (almost) maxed out Jackdaw. Everything except el Impotulo took me one try. **** El Impotulo. Took me probably 10 tries. I would spend the entire battle maneuvering only for it to turn on a dime and ram me. I finally learned to spam fire barrels (only time in the whole game I used them) to keep it away.
The one that annoyed me the most was the one that liked using mortars. Even though I could dodge 90% of its mortars, sometimes it would hit me and wreck my ship. And it liked ramming and jumping on me as much as Impoluto.
Love is what gives you true strength. Pokemon, Naruto, and Grand Theft Auto taught me that.
One try for all, had everything maxed except cannons and mortars (elite mortars really would've helped :l). None of them are particularly difficult, El Impluto repeatedly ran away from me when I threw down the fire barrels and I'd light him up with everything I had every time he did. There was a couple times he'd just ignore the barrels and proceed to ram my ship though, but I didn't hurt that much. Every other ship was easy.
The bottom two I was able to knock out after a couple tries, but am still having trouble with the NE pair of ships. I can knock one of them out pretty easily by just spamming the ram and chain shot, but then the other ship goes psycho and just destroys me. I haven't even tried the NW ship yet, but from everything I've heard it sounds like he is the real kicker.
I took out every legendary ship on my first try, except for the two in the NW. That took me 5 tries.
The legendary ships are absurdly easy, you really have to max your ship.
My strategy for every ship now is to ram them, heavy shot, then the holding Y. Rinse and repeat. Ramming has to be the most powerful attack, followed by the heavy shot. The heavy shot is basically a shotgun, so ram from the side/corner to put you in position to land off one or two heavy shots.
Mortors are pretty weak except for small boats. I did have to rely on mortars when facing the two legendary ships. I was definitely try to keep my distance from the 2nd ship, who was so far away after I killed the first one that it did not go try and ram me. Also, it was low on health and I was able to ram it.
Best part? You unlock a better way to ram enemies. The way the 2nd ship tries to ram you, you get that crazy speed when killing all legendary ships.