The Opening Act - 10G (Original Achievement)
To get this Achievement, simply clear Green Hill Act 1.
It doesnt matter how fast you are or what Rank you get. Just get to the flag at the end and touch it.
Below is a video of mine that shows how to get this Achievement, as well as three others.
The Ring King achievement in Sonic Generations worth 25 pointsReach the goal without dropping any of the rings you collected in GREEN HILL Act 1.