You must perform 10 consecutive leapfrogs in a row. These are done by pressing when positioned right next to a zombie. You can easily get this right at the start of the game during the Prologue. Kill all but one of the zombies and just keep leaping over him by pressing until the achievement unlocks. Make sure you don't do any other type of dodge because it will reset your leapfrog counter for the achievement as it needs to be 10 in a row. Also your counter will reset if you unintentionally kick a zombie while doing a leapfrog jump.