Kao the Kangaroo

Story-related; cannot be missed.

The Oldest One is the boss of the third area, and is even easier than the previous boss. 

For the first phase, keel running in circles to avoid the projectiles, grab a fireball, punch any mirror, then get behind it and stay there until his shot deflects back at him and stuns him, so you can run in and hit him. Repeat this two more times, taking note that the mirrors move now, so you'll need to stay moving behind them.

For the second phase, he splits into clones. They'll appear around you when you're in the center. While each clone has a snowflake above their head, the ice that you can get from a totem just turns them into an ice cube for a bit, so it's really not that useful. Instead, look for the clone acting different from the other, and just go up and combo him. The others will disappear once you hit the right one. Repeat this process, rolling around until you identify the real one, and he'll be gone very quickly.