Beat the game on easy... but why not get the achievements for beating it on hard and beating it on impossible via a cheap trick?
Start up the game and play on easy all the way through. Once you reach the last boss (Sonny), you will want to wear him down as you would any other boss fight.
Just before you deal the last blow, pause the game and change the difficulty to Impossible. You will unlock all of the difficulty achievements at once even though you played through 99.9% of the game on the easiest setting!