Here is the full list of all 13 Flem achievements.
Beat Hardcore Mode.
Find the Land Of Eternal Tissue.
Survive the Winter.
Get all Gold Medals.
Survive the Fall.
Get 30 Gold Medals.
Survive the Summer.
Get 20 Gold Medals.
Survive the Spring.
Get 10 Gold Medals.
Die 10 times in one level.
Get a Gold Medal.
Survive the Great Sneeze.