To get this achievement, You have to Clear a regular stage as Super Sonic.
To do this you MUST first complete the game once. Then after completing it, you'll unlock Super Sonic as a skill for Classic sonic and for Modern Sonic. Simply head to the skill shop and equip it (skill points req.100) and head to green hill zone(or your favorite I did it on green hill) Gather 50 rings and the option to press the Y button and turn super sonic will appear. BUT you have to finish the level as Super Sonic. When you Super Sonic your rings reduce and as super sonic(modern) if you choose to boost your Rings reduce faster.. So to insure you do this I would wait till just about the end of the level to go Super Sonic.
Thanks to redjarman for pointing out that Modern Sonics rings reduce faster, and thanks to HarrySon9 for clearing up that you don't have to buy it.