This achievement is for completing one of the secret rooms.
It can be found in level 7 "Reptilicus", at the very beginning of the level. Simply walk left instead of right and you will see the triangle that starts the bonus leve. Jump right into it.
You will have to collect 15 of the smaller icons and then the door on the right opens up and you can finish the level. Switch your characters with
to collect the small icons and evade the black stuff. You retry as many times as necessary.