This one is pretty straightforward.
You have to finish the game once, and after the credits roll and you return to the title screen/save select screen, just pick the save slot you were using and it will present you with 3 options:
Continue
New Game+
Circuit Mode
Just pick New Game+ and play through the game again.
It's not difficulty-specific, so you can pick Easy Mode if you want.
Just finish the game again, and at the final results screen, this Achievement should pop.