This can be obtained when taking control of your first ship (Adrestia) during the mission 'Learning the Ropes' (at the very beginning of Episode 2).
First the enemy ship needs to be damaged enough for the final blow to be from ramming into it. This can be done by firing arrows (Hold LT + Press RT) and javelins (Press RT) at the enemy.
When enough damage has been done, line up your ship with the side of the enemy ship and hold (A) to ram into it.