After you beat the game, you'll get some cutscenes. Watch them or skip them, it's up to you. Once they are finished you will get the Credits. They are unskippable. Watch TV or use a different App until they finish (TA anyone?), you will get the achievement automatically.
If you want to watch the cutscene I suggest using Snap-mode rather than fullscreening away finding something else to do since Microsoft Studios had the bright idea of getting rid of Snap-mode.