This is one of the simpler achievements in the game. All you have to do is start the story mode and then play as Patroklos in the very first episode of the game. You just need to beat his opponents in all three matches, which are done one-by-one. As soon as the episode is done, the achievement should unlock automatically. This should only take one try, but if it doesn't happen the first time, just do the fights all over again until the episode is completed.