Dormammu destroys Pheonix.
Especially in his realm. He "IS" reality in his realm, controlling everything.
He has an eternity of experience that goes hand in hand with his power.
Dormammu is a minor mystical entity who resides in a small pocket dimension within the 616 continuity. The Phoenix is the life force of creation. Know your stuff son
Listen. If you are going with a Phoenix Avatar, then Dormammu can win. If you're going with the whole damn Phoenix Force, Dormammu doesn't stand a chance. Example. Rachel Summers is an Avatar, and tapping into it has knocked down Galactus, and stopped the energy blast of a Shiar Warship that was supposed to destroy a hundreds of miles of radius. PF wins instantly of course, but see... PF wouldn't have ended up in a situation where it would have to fight Dormammu... its an entity... Now the avatar.. and I say rachel cause she's the best example, would go 3/10 with dormammu in his own realm.
__________________ Quotes from Hia8:
"I claimed that the science is sometimes faulty."
"You don't understand. This is fiction. That means none of this stuff really happened."
"There is no writer to purposely ignore a character's natural ability just because it suits the story."
"in some cases because the writer knows that Character A will dominate Character B easily and refuses to allow this to happen for the sake of the story."
Dormammu would lose more often than not to Phoenix. He's been bested by lesser beings more than once in his own realm while he had the Flames of Regency. Unless Dormammu has prep to do something like he did in GOTG, he's going to fall to Phoenix. All it'll take is for Phoenix to destroy the barrier that keeps the Mindless Ones in place.