The Banner Saga

This achievement will come easily, and you can grab it a little after the start of the game. In order to get the achievement, you need to defeat an enemy with a Warmaster. This can be done with Hakon (the dark haired varl) in the first chapter of the game.

There will be multiple times when you will initiate combat, so all you need to do is make sure that the Warmaster gets the final kill. You can let your other allies damage the target first, just as long as the final blow is by Hakon. Warmasters are also incredibly powerful, so it'll be easy to inflict damage.