Egg Totem
From Sonic Retro
Egg Totem (エッグトーテム) is the boss of Music Plant of Sonic Advance 2. This boss is a tower consisting of Robotnik, three spiked discs, and a platform with wheels. Every once in a while one of the discs comes off. The discs sometimes show small bullet-shooting cannons that can be destroyed. When destroyed, use the now-safe platform to get to Robotnik. Once defeated, you rescue Tails, and can then play as him.