Rallyhn
i would say Dark Falz on Phatasy Star Online episode one, he kept turning invincible alot of times
Feh, he gets his a** handed to him by the true dark god of the PS universe, PSIVs Profound Darkness. This little b*****d changes shape twice over the course of the battle-and gets nastier each time.
Starts out as this big, pulsating mass of red flesh with spiky mouths all over it. He's pretty easy to manage, except when he uses Shadowbreath, which hits one character pretty hard.
Then he turns into a big black monster with spines, and starts making with the party-effect spells (Anothergate, Lightshower, and one other that escapes me). You're pretty much guaranteed to lose a character here.
And after that, he gets serious. The music changes, the background changes, and PD turns into this freakish humanoid figure with giant spiked wings, flaming hair, and a huge green cape of light. Uses a spell called "Cancelling" which negates Deban, Warla, Blessing, Barrier, and any other defensive or status-boosting magic you may have had up. Then, while you're trying to reconstruct your defenses, he goes and uses the Ultima-crushing "Megid," which, naturally, is far more powerful than the version the main character can use. On top of that, he has an incapacitating sleep spell called "Evil Eyes" which works way more often than it should, and the ability to blast any one character for ~190 damage.
And that last form takes FOREVER to beat. By the end you're left wondering "Jesus, how much longer do I have to wail on this thing until it goes down?" I must've died 10 times against that rat b*****d before I finally beat him.
Oh, and just to add insult to injury, you've less of a chance at beating the PD at level 99 than you do at ~65, or any level up to and including 98. Once you hit 99, your characters start LOSING THEIR ABILITIES, starting with the most powerful ones (Megid, Legeon, Positron Bolt, Rayblade).