After completing the first four seasons, return to the top right location (the bear).
Now go as far right as you can, and you will come to a steep cliff, with a mushroom/plant spawn underneath.
Retrace your steps to find a bug and chase it into the mushroom spawn. Now climb up and you should be by some tombstone things. Bark (X) to activate them and a cutscene will play, showing you a teleport/zoom.
Retrace your steps to this and jump on it to move to a new location. Freeze the waterfall and jump up it to get to the other side. Move down and on your left in a gap is what looks like an animal den/bed. Enter it and a cutscenes will start. The achievement will unlock at the end.
Good luck!