Legion isn't an XMan and isn't more powerful than Phoenix Force Jean Grey.
If we're quoting wiki:
The Phoenix Force is an immortal and mutable manifestation of the prime universal force of life and passion. Born of the void between states of being, the Phoenix Force is a child of the universe. It is the nexus of all psionic energy which does, has, and ever will exist in all realities of the multiverse, the Guardian of Creation, and of the dangerously powerful M'Kraan Crystal. The Phoenix is among the most feared beings in all of existence — having the power to cut and re-grow any part of the universe, as well as destroy it entirely....
If we're quoting wiki:
The Phoenix Force is an immortal and mutable manifestation of the prime universal force of life and passion. Born of the void between states of being, the Phoenix Force is a child of the universe. It is the nexus of all psionic energy which does, has, and ever will exist in all realities of the multiverse, the Guardian of Creation, and of the dangerously powerful M'Kraan Crystal. The Phoenix is among the most feared beings in all of existence — having the power to cut and re-grow any part of the universe, as well as destroy it entirely....
List of hosts- Jean Grey: The most powerful and complete Phoenix Force and Host combination. Together they have become the White Phoenix of the Crown.[68] Death has stated that Jean is the rightful owner of the Phoenix Force, and that she is not just a host, she is the Phoenix and the Phoenix is her.[69]