To get this achievement you must beat all bosses as Super Sonic. Every Boss Act and well as Death Egg mk.II Zone Act 1 must be beaten as Super Sonic for this to unlock. To get Super Sonic you must get the seven Chaos Emeralds beforehand.
For the Chaos Emeralds see All Chaos Emeralds Found!.
When you have acquired Super Sonic you need to gather 50 rings and press transform. Using Tag Combos with Tails will cancel out the Super Sonic state. If you still have over 50 rings just press again to transform.