Assassin's Creed Odyssey

To unlock this achievement you need to destroy any epic ship. The first chance to do this is when you first gain the use of the Adrestia. In the area where you destroy the Athenian blockade before getting to Megaris, you can find 'Helios the Lucky Drunk' sailing around. This ship is a level 9 and can prove quite a tough fight so early on with little to no upgrades, but it is possible. My tactic was to try and ram into the side of it as that was by far the most damage I could do. Though if you start to fumble near it after ramming and lose manoeuvrability, the arrow shots from the Helios are devastating and do massive damage, so be careful not to stay near it for too long. Fire your arrows whenever possible to do some damage, then line up for a ram once you're in position. After either destroying the ship, or boarding and killing everyone on board, the achievement will unlock.

Other than this ship, an 'Epic' ship is just a ship controlled by an important character, so either a cultist, mercenary or occasionally an enemy leader during a sea conquest battle.