Sonic Adventure/Sonic/Ice Cap

Ice Cap
The Ice Cap contains the next chaos emerald, but it is locked. To enter, you need to find an item in an alley near the Casino, and bring it to the Mystic Ruins. A cutscene should show a hole breaking open in a cliff, which you should enter.

After finding the Chaos Emerald, wander around in Mystic Ruins. As you approach the lake area, Sonic and Tails will meet Knuckles. Sonic greets Knuckles but Knuckles just gives him crap. He tries to attack Sonic with his punches but, of course, he misses. Knuckles wants the Emerald Sonic has because he thinks it's a piece of the Master Emerald (Robotnik tricked him; he is on a search for all the Master Emerald pieces to restore it). Sonic refuses to give it up and they get into a fight.

Knuckles
Knuckles is also nothing to worry about; he's easier than the previous enemy. His attacks are a simply spin attack, and a glide attack. The easiest method to defeat him as Sonic is to jump, and use the homing attack. If he is attacking at the same time, you will bounce off him, but you will otherwise land a hit.
 * Difficulty: Very Easy
 * Damage: 3 hits

Once you make the third hit, a cutscene commences with Sonic and Knuckles dropping their chaos emeralds, and Dr. Robotnik taking them. The emeralds are then fed to Chaos.

Chaos 4

 * Difficulty: Medium
 * Damage: 5 Hits

Chaos takes on a swimming form, and to survive, you need to keep on the lily pads. Chaos tries to damage you either by knocking the lily pad over, or by launching a shockwave at you (which may be jumped.)

While swimming, chaos can't be hit, but it will surface from time to time. When it does, jump into the air, perform a homing attack, and quickly move to the nearest lily pad.