With a ship, take on superdreadnoughts in surface Alyria. One spawns near 1183, 1414, 2153, 1362, and around 258, 586. Each time you sink one of these, there is a small chance a line will appear: "As the mighty super dreadnought cleaves into multiple parts, its mainsail stays upright, a curiousity on the high seas." This means that the boss has spawned. Park your ship in safety and walk to where the ship sank and go up.
Encounter: