You can only achieve this once you've beaten the game
Once you have beaten the game go to chapter 26 and then go down the ladder to the left and then once you are at the bottom of the ladder go right down the deep dark mysterious cave into the depths of darkness where you will find nothing but death and a trophy.
For more info on the trophy use this guide by aXHNX
. It's one of the best ones out there as it clearly tells you what to do at each stage in the secret level: