Flame Mobile

Quite a step up from the easy mid-boss, Angel Island Zone's boss, Flame Mobile (フレイムモービル), involves hitting Robotnik's eggpod eight times, while avoiding the two (or four, if playing as Knuckles) fireballs shot from it. At both ends of the arena are bottomless pits, and Robotnik will disappear behind the waterfall every now and again so you'll have to time your jumps. After your points tally go right, you meet the malicious Knuckles again—he laughs and hits a switch which makes the bridge you're on collapse, and you fall into Hydrocity Zone.