Sonic & Knuckles

I got this achievement without fighting a second boss. I kept running to the right, and ended up finding a Secret Zone, it looks like the main path is heading right and there's another path going down and to the left, if you keep heading right, you find the Secret Zone which leads to the Sky Sanctuary.

This involves completing both acts of the Lava Reef zone - you will be awarded the achievement after defeating the second boss, if you're really struggling - use the save feature.

You can do this as either Sonic or Knuckles

Hot Feet - 15G (Original Achievement)

Difficulty: 5/10

Lava Reef is the 4th Zone in the game and this achievement can be obtained while playing as Sonic or Knuckles. The boss here is only for Sonic to fight. The boss is a machine that roams from the left side of the screen to the right in a pool of lava. You will want to stay in the middle and jump from platform to platform to keep from being pulled into Robotnik's machine. Robotnik himself will stupidly shoot metal spike balls to try and hit you, but they will land in the lava and get pulled into him. This is the only way to damage him so just keep moving and dodging. '

Note: If you watch the video, I show off a Fire Shield right before the boss, with this, you can walk on the lava without a problem, just make sure you avoid Robotnik and the falling spike balls or else bye bye Shield.