Who do you think can defeat Dormammu in his own realm? In his own realm, Dormammu is absolute. Few could beat him in his own realm, but what of those who can?
Archenemy already did, i mean making him flee from his realm to prevent his death should count as a win shouldnt it?
Really you dont need to go all the way up to Living Tribunal, fed Galactus and Celestials could do it.
Also while Dormammu gets more powerful in his realm, he is by no means unbeatable or absolute there.
@veitha: Well at least on Limbo, with her sword, Magik was portrayed as being superior to Dormammu, so that sort of helps. And Dormammu actually has outright taken over Mephisto in hell, had him helpless without having to lift him finger.
In short what i am trying to say is while Magik could probably win , in a realm that squarely favors Dormammu , he should get the upper hand most often than not.
Is that even allowed in a fight? Also:
Bolas is too good for the likes of cancel =P
In all seriousness though, Bolas is a mostly legit answer here since he is likely the most proficient Planeswalker at negation magic to exist up to this point. Dormamu with no Magic vs an Immortal God-entity with boat loads of magic.
Thanos. Especially with either the Cosmic Cube or Infinity Gauntlet, but I think he could do it without.
Thanos would get killed quickly if he didnt have a cosmic item to use against Dormammu especially when he's in his own realm
Current cyclops.......... Sad but true, cyke downed him with one shot in the last uncanny issue
that was just so horrible I really hated how weak they made Dormammu. Yes he wasnt in his realm but Dormammu isnt some low tier villain out of his realm he still pretty strong.