The Doomsday Zone
From Sonic Retro
The Doomsday Zone is the eighth zone in Sonic & Knuckles (the 14th zone in Sonic 3 & Knuckles). Only Sonic may play the level normally, and only after collecting at least 7 emeralds (i.e. only in Super or Hyper form).
The Doomsday Zone is set above the planet, in asteroid-filled space. You must fly through the asteroid belt and fight Dr. Robotnik to recover the Master Emerald and foil his plans again!
Because Super Sonic must play this stage, you start out with 50 rings instead of 0, and you lose rings at the traditional Super rate of one per second. There are extra rings along the course of the level, but if your ring total drops to 0, you fall out of super form and lose a life.
To defeat the first form, dubbed the Egg Missile, drive the missiles it fires into the cockpit of the ship. After 8 hits, Eggman returns in the Egg Bomber, resembling the Giant Robotnik boss from Sonic 2. Keep your ring supply up by hanging back when needed, but when you feel comfortable with your ring total, hold right and hit any of the face buttons to begin ramming into him. Hitting his bombs and the asteroids slows you down. After 8 hits, the Master Emerald is freed and Eggman is beaten again. Congrats!
The Doomsday Zone set a precedent that would become the template for most final boss battles in future games. Among them: