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.
You started the game once with each character.
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.
Enter a Bonus or Special Stage.