Iron Man is seen as one of the most powerful heroes in the Marvel (cinematic) universe. But there are some adversaries that even he could not overcome
1. Static Shock
Virgil Hawkins is able to manipulate metal objects and their electricity around him. His electricity is different than normal electricity so, unlike Thor's, his would not overpower the Iron Man suit and Static could shut it down. Also he has his hover plate, so he could also fly with Iron Man for aerial battles.
2. Red Hood
While Iron Man may use cautionary shots at first, Jason Todd (a.k.a. Red Hood) is a cold blooded killer who will not hesitate to kill Tony the first shot he gets. His arsenal is equal more or less to Iron Man's and he is more agile because he doesn't have a suit holding him down. He would have a disadvantage aerial-wise but he does have a helicopter.
Scott Lang or Hank Pym, literally anybody who dons this costume at one time or another, would be able to take down Iron Man. Think about it, there doesn't even need to be a fight! Ant-Man could shrink down, start disconnecting wires and guns and thrusters, and the next time Tony goes to take off, he explodes!
4. Aang (Avatar the Last Airbender)
Now, if we want to get technical, Aang is a superhero. He controls four elements and bends them at his will. If he wanted to blow Iron Man out of the sky, he could do it. If he wanted to freeze his suit over and make it malfunction, he could do it. If he wanted to sear the suit to Tony's flesh, he could fucking do it. Aang is probably the most powerful on this list and he could take down Iron Man with ease.
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