Another easy achievement. Start as the Highlander, and play against another "Local Player" you control - also as the Highlander. As you play, just make sure they land on each other's spaces, and have the player 1 Highlander win the fight against the player 2 Highlander.
This is made easy by the Highlander's "Charge!" ability, which adds an optional +2 to their attack roll. So don't use it for the player 2 Highlander, but use it for the player 1 Highlander, always stacking the odds in your favour. You can also use your Fate points to reroll either Highlander's attack rolls if necessary.
Losing lives as the player 2 Highlander in other ways can help, if the option presents itself. Just make sure that the player 1 Highlander takes the final life of the player 2 Highlander in a regular character Battle, and you'll unlock the achievement.