Kao the Kangaroo

Story-related; cannot be missed.

The Eternal Warrior is the final boss of the game, and definitely more involved than the other bosses, both in terms of a longer fight and more stuff that'll hurt you. 

In the first phase, you'll get a group of four enemies spawned around you. Beat them while being careful of the warrior's chain attack. Once they're dead, he'll raise up three platforms with a blue object on the highest. Jump to the top while being careful of his chain, because just touching it hurts you. Pick up the blue object, aim, and throw it at him, while still being careful of a chain attack. After four cycles phase one will end. 

Phase two is markedly easier. His only attack is his chain attack, and as long as you keep moving around the circular arena it won't be an issue. There are three crystals you need to hit, and each requires at least two different elements. There is a totem for each element on the ground. Run/roll around and grab one of each element, then head to each crystal where you'll need to use at least two to reach it, typically unfreeze it, then punch it. After punching one, hop down and punch the boss, then repeat for the other two crystals until he's beaten.