After "The Local Flavor" you must go to the Cathedral on the north-east of the map.
Use the intercom on the front gate and you'll be told to get the seed of a giant plant.
Make your way to the swamp, find the giant plant and use it.
Now is the funny part of the DLC ^_^
When you're back to 'normal", go to the Cathedral, talk to the man on the intercom again and then to the man in the courtyard to unlock the achievement.