This achievement requires you to activate the expansion "The Reaper" and then choose to play as the Genie character. It's best to play with another, second character you also control("Local Player"), and they need to be able to have followers. (So don't choose the Ninja.)
Run around with both characters until one of them eventually draws the Genie follower card from the deck. Let the second character pick it up - don't pick it up with the Genie character. Now that your second character has the Genie follower, battle them with the Genie character.
When the Genie character wins the battle, you're able to take a follower instead of an object, gold, or a life from the opponent. So choose the Genie follower as your spoils of war and the achievement will unlock.