Full list of all 12 Sonic & Knuckles achievements worth 200 gamerscore. It takes around 4-5 hours to unlock all of the achievements on Xbox 360.
Complete the game as Sonic.
Complete the game as Knuckles.
Earn a Continue in either a Bonus or Special Stage.
Collect all seven Chaos Emeralds.
Collect a Chaos Emerald.
Finish the Lava Reef Zone.
Finish the Sandopolis Zone.
Finish the Flying Battery Zone.
Finish the Mushroom Hill Zone.
Collect 100 rings.
Start the game once as each character.
Enter a Bonus or Special Stage.