Achievement is super easy.
There is a Boss Battle mode for Hero, Dark and All
Simply beat all the Hero Boss Battles, Dark Boss Battles and All Boss Battles in under 1 hour per mode and you will unlock this.
The All Boss Battle Mode uses all the bosses from Hero and Dark stories as well as the Final 2 Boss fights. They are fairly easy battles since you have completed them all already. Just be sure you have lives in case you fail. I know I hated the Tails Vs Eggman ones as I had trouble avoiding the special move.
It really doesnt take much effort to beat this. You can probably beat them in 10-15 mins for the individual Hero\Dark Boss modes. The "All" Mode will put you close to 30 mins