For the Absolution achievement, you must beat the game on one of the Professional difficulties. These difficulties are Hard, Expert, and Purist. Any of the three will do but if you are simply interested in getting this achievement, go for hard.
For more help on beating the game, be sure to check out our walkthrough. Remember that if you're having some trouble, you do not have to get a high score on each level. In fact, you could just shoot your way through the levels though on higher difficulties, this will prove to be quite difficult.
