In order to earn this achievement, you'll need to play the main campaign and reach the eighth episode. Once you arrive at that point, all you have to do is defeat your opponent and finish the episode. If done successfully, the achievement should automatically unlock for you. It can be tough, due to some of the opponents being cheap, such as Nightmare, but just be as aggressive as they are and you'll be fine.
That's all you need to do here. Reach the eighth episode and complete it in order to earn this achievement. Good luck!