X-Men: 10 Things Everyone Gets Wrong About The Phoenix
5. The Phoenix Is All Powerful
While the Phoenix Force is a power predating life itself and being born at the birth of the universe and is nigh immortal... that isn't to say that it's invulnerable.
The Phoenix Force has been shown to be hurt by the powers of the Iron Fist among other things. Likewise, as hinted at earlier, it can be susceptible to mental manipulation and powerful telepaths.
Recently, in a dust up with Thor, it's even been shown that the Phoenix Force is susceptible to a good whacking by the mighty hammer Mjolnir.
But moreover, when the Phoenix is merged with a host, if anything it seems to become even more prone to damage. As despite surviving falling into the sun, when the Phoenix was last merged with Jean Grey, Jean eventually found herself getting killed by an electromagnetic pulse from Xorn (who was at the time posing as Magneto... because comics).
Apparently this caused a 'cosmic scale stroke' (that's what she said) and the Phoenix's immense power seemed unable to stand up to it.