Here we go.
Superman would win.
After a sun-dive he becomes nigh-invincible. Nothing can pierce his skin. Nothing will hurt him and his strength is increased dramatically. I'm not talking about the strength to cause an earthquake. I mean literally Earth shattering strength. The power to rip a star apart with his hands. He could literally juggle planets if he wanted to after a sun-dive.
The only thing Phoenix has against Superman is her telepathy. She could affect Superman's mind but she couldn't destroy it. That'd be using telekinesis. Causing physical damage. After a sun-dive, nothing could hurt Superman. Not even a punch from God himself. Phoenix's demolecularizing powers? Wouldn't work. It's not magic. Superman's invulnerabilty would beat it.
And so, since Phoenix really has no other upper-hand over Superman, he would then proceed to take her out (Not kill her, but to the point to where she's no longer a danger.).