Simply clear the game on "Easy" Difficulty
But watch out ! This achievement isn't unlocked if you finish the game on Normal/Hard/Extreme
If it doesn't unlock at the end of Episode 6, check you Area Completion List
Much people forget the 2nd Area in 1-2, it's a hidden area, at the beginning of 1-2-1, there is a hole, hidden by some trees on the right. Finish this hidden area AND the following one to unlock the achievement