Defeat Harlow is one of the secret achievements and trophies available in Dead Island: Riptide.
The name is pretty self-explanatory. You don't need to do anything special to unlock this, just keep playing through the game. Once you reach Harlow, fight her as you would. She's a difficult fight, as until the final part of the fight she is invulnerable, but after you land the final blow and the cutscene plays, you'll unlock this.