SONIC X SHADOW GENERATIONS

This one can only be obtained after completing the main story. After you respawn from completion, you'll see a black doom eye infront of you which activates the "Doom Zone". Activating this will unlock the remaining challenge acts and then it's a simple case of pausing the game and choosing the challenges in the "challenge list" tab if you don't fancy travelling between challenges. Doom Zone isn't permanent and can be turned off anytime but you need it activated if you're going for this achievement.

Be forewarned that these are really difficult, especially when going for the S ranks.