That was my original idea . She defeated Xavier in mental combat.
Xavier frove many people with (what it looked like) no problem at all.
Jean is a telepath as well. And with the amp she toys with the ability.
Clark will appear on the scene. She sees at the speed of thought his intentions and attacks.
Thing is Clark is SOOOO fast. That he could very well override the telepathic assault and just let lose with the punches.
Which opens the next question. Did Phoenix die because of her lust for Logan???
She held back against Logan when he defied her. She burnt him just enough for him to heal, but not enough for him to be completely destroyed like she did with Xavier. (turning him completely to dust.)
She doesn't have such a dormant lust for Clark. She just wants to kill him.
But speed is the issue. And if Clark goes "all out" he'll apply more damage to Jean than Logan did.