Dark Yabu from Vexx. He's almost impossible to beat and you have to be lucky to actually beat him. Why? Well, he has three phases. The first phase he sends out his shadow creatures to attack, next he'll throw down meteors. and then he'll summon his minion while throwing down meteors. Second phase he forms into a spider and he shoots fireballs out of his mouth and he does a shockwave attack. You have to hit his weakspot 5 times without dying and each time you hit him, he gets faster and faster. Third phase he returns to his original form and brought up pillars for him to stand on. He sends his only wave of his shadow creatures and after that, he throws fireballs at you and you're suppose to knock them back to the pillar he stands on. Once the pillar's destroyed, he hop on to another pillar while the arena around you breaks down, and you keep on doing it until he has no pillars and able to knock the fireball back at him, thus defeating Yabu.

Summary:

1st Phase was difficult
2nd Phase was nearly impossible
3rd Phase was surprisingly easy