I am assuming you are using "far-fetched" in one of two ways - questioning my recollection of the events, or questioning Dark Phoenix's powers.
I am now positive it was "What If Phoenix Lived." I also have "What If Jean Grey Rose Again." On the cover of that one Jean Grey is in green and gold. That is the incorrect issue.
It was the issue in which Dark Phoenix slipped out one evening to "feed." She consumed a sun. She kept saying to herself "no one will ever know." She flew back. Xavier knew what she did all along. At the end after the Phoenix Force consumes the universe, you see The Watcher on the last page observing.
If questioning her powers, do notice I said consumed a sun. Gentlemen, Galactus consumes planets, and needs equipment in order to convert it into the requisite energy type. She consumes SUNS and instantly.
In the issue Xavier essentially said "We cannot allow you to do that again." Dark Phoenix got that glint in her eye and responded by asking "who are you to allow ME to do anything?" She then killed Havoc, Nightcrawler, Colossus, and Wolverine. Xavier engaged her in a test of willpower. You see them both squinting their eyes. It then says Xavier realized Dark Phoenix was merely TOYING with him. You then see Xavier doubling over and falling forward out of his chair. Cyclops then screamed "Jean NO!" and hit her with his HARDEST blast. It only knocked her off-balance. She sent a blast of energy through his chest.
If you are STILL questioning her powers, Xavier is one of the world's foremost telepaths. She TOYED with him. She beat him without even TRYING. Cyclops hit her with his HARDEST concussive beam. It merely made her lean a little to the side.
"Dark Phoenix (and of course, Phoenix) had extreme telepathic, telekinetic, and pyrokinetic powers. In addition, she also exhibited the following abilites at various times: altering the molecular structure of items, not needing to breathe, and in general whatever else she wanted."
Pay special attention to this part: "and in general whatever else she wanted."
Other quotes.
"The Phoenix is the most powerful force ever to be held in mortal hands at ANY comics company."
"Dark Phoenix tears through star systems and galaxies in the blink of an eye."
NOBODY drew her better than John Byrne and Paul Smith did. Honorable mention to John Romita Jr.
LoL, better ask somebody! Remember Rachel beat Galactus with The Phoenix Force. Dark Phoenix was FAR nastier.