From Sonic Retro
The Egg Saucer (エッグソーサー) is the boss of Sonic Advance 2's Sky Canyon. And what a boss it is. The first time you get to this one as Sonic, Knuckles (who was again tricked by Eggman this time) is driving it.
Eggman/Knuckles, a laser-shooting mech, and a hand are spinning around on a wheel. The goal is to hit the laser-shooting mech until it breaks (takes 4 hits), then hit Eggman/Knuckles until it breaks. But watch for the hand -- if it hits you, it counts as a crush and you lose a life; if you touch it after it has already tried to smash you, you'll lose your rings. Eggman/Knuckles is on a platform with spikes on the bottom, but these shouldn't be a problem if you know how to move.
After defeating this boss for the first time with Sonic, an oil-covered Knuckles comes out. The level ends, and a cutscene starts to play: Sonic quickly understands that Eggman had tricked Knuckles again, and the echidna feels really ashamed for this. Sonic then continues on his way, leaving Knuckles behind (who right then becomes a playable character).
Curiously, in this cutscene Knuckles' eyes are cyan rather than their proper colour, violet.