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