By the end of getting all other achievements you should be at about 70-80% through this achievement.
SOLO IDLE METHOD
Load up the level "Water". Look in between the row boat and the tall rock on the left in the water and place something on your "W Key" (Move Forward Key) that will hold it down for you. Once you do you character will automatically walk forward into water then will re spawn in the same direction you are looking and keep walking forward in the same path. I found that doing it on "Water" is the fastest since I think when you go down the hill while walking in the water you get a speed boost possibly and gets more distance quicker, also you re spawn faster when you drown instead of falling off the die of the map.
Doing it this way Ive gotten about 1% progress a minute. So after doing all other achievements this wont take you long to finish.