Lava Reef Zone
From Sonic Retro
The longest level in a Genesis Sonic game, Lava Reef Zone is an underground labyrinth of crystals, magma and general chaos. There are four acts to this level, although the fourth act with Sonic is just a simple boss fight against Knuckles followed by a cut scene. The first act is a mine type level, the second act is a crystal wonderland with flamethrowers and spikey things and the third act (only playable with Sonic) is a giant boss fight with Dr. Eggman.
To beat the boss in the first act, stay in a corner and dodge the lasers from the orbs. Try to hit them if you can, as that will eventually destroy them and that will be one less distraction from the real boss. A giant hand will follow you around, but when you see it stop, or start blinking, get away. It will come down trying to squish you, which at this point you attack the hand while it's down. Hit eight times to defeat... the orbs will die in three or four.
To beat the second boss, avoid being on a platform that Dr. Eggman's missles hit. You'll eventually get to waterfall of magma. Follow your way down and you'll get to a boss with Eggman in a tank type machine. If you can, get a fire shield before this, or go super, as it will make this a LOT easier. You'll want to jump from platform to platform to avoid falling into Dr. Eggman's ship. The machine will shoot mines into the air and if you just keep jumping platforms, the mines will hit Eggman. This has to happen numerous times before this boss is defeated.
The third boss in Hidden Palace Zone is Knuckles and he can be hit from the back when he's flying or by being jumped on top of in the corner when his guard is down.