yeah, anti-tank scales, and rapid healing giving him the ability to survive a nuke are a bit OP. That, combined with the strength and size make it kind of too powerful to really deal with.
I've got no problem with the human form. The size and strength of the dragon form wouldn't be a problem if it could be injured more easily. Maybe if it took a tank shell to penetrate the scales? In videogame terms, his defense is high, but his HP is extreme at that size.

I put Phoenix in there as a kind of "ultimate enemy", the final boss of the game, someone we'll barely survive the first round with, and will only be able to beat after severe training and coming up with a superior strategy that exploits her weaknesses.
Even my character, if he grabbed her and tried to copy her powers, would only be able to get a mostly depleted copy of Jean Grey's powers before getting kicked off of her. Rogue, who's pretty OP anyway, wouldn't be able to steal her power either without hurting her severely.