On the first level "Nagoya City" after your characters have met on the rooftop and the rest of the map opens up, you will make your way to a convoy of wagons stuck on a bridge. On the right-hand side behind the buildings with the samurai standing in front of it, are stairs leading to the water. Jump into the river and swim to the right until the camera won't follow any further, you will be teleported to the other side of the map and the achievement will unlock.