For the big finale of its first three phases, the Marvel Cinematic Universe adapted the classic Infinity Gauntlet storyline from the comic books. The proceedings truly kicked off with Avengers: Infinity War, in which Thanos gathered up the Infinity Stones and battled against Earth's Mightiest Heroes on several different fronts. And it was on one of those fronts, Titan, the MCU's biggest villain faced its very first superhero: Tony Stark/Iron Man.
On Titan, Iron Man joined the Guardians of the Galaxy, Doctor Strange and Spider-Man in a bold plan to defeat Thanos, but it rapidly went bad. As a result, Tony engaged his most powerful armor in a battle with the Mad Titan -- and he lived to tell the tale. However, the case was much different in the comics.
In fact, when Iron Man faced Thanos on the Moon, the Mad Titan killed Tony Stark without even lifting a finger.
The 1991 Infinity Gauntlet series by Jim Starlin, George Pérez and Ron Lim was different than Avengers: Infinity War in its ending. Instead of ending with Thanos' snap, the series started with it. It was only after he made half of all life in the universe disappear that the remaining Avengers banded together and launched an attack on the Mad Titan, who at the time was waiting for them on Earth's Moon. With the Infinity Gauntlet already in his grasp, Thanos essentially had the powers of a god, and the Avengers had little hope of success.
The inevitable failure of Earth's Mightiest Heroes becomes very clear when, despite their best efforts, Earth's Mightiest Heroes are picked off by the Mad Titan one by one: he rips Vision apart; he cuts off Cyclops' access to oxygen; he transforms Wolverine into rubber; he pulverizes Quasar with an energy blast. As for Iron Man, well Thanos doesn't even deal with him personally. Instead, it's Terraxia who fights with Tony Stark.
Terraxia was a creation of Thanos. With his love for Death not reciprocated, Thanos used the powers of the Gauntlet to create Terraxia in Infinity Gauntlet #3. She was essentially created to be his devoted girlfriend, and she leaps to his defense when the Avengers attack in issue #4. After Iron Man fires his repulsor blasts, she knocks him off-course. And next we see Terraxia, she brings Thanos the severed head of Iron Man. Not only does Tony Stark die at the hands of Terraxia in the comic, it also happens off-panel.
Therefore, in the original Infinity Gauntlet comic, Thanos never directly faced Iron Man in battle. Instead, it was his creation who took care of the Armored Avenger.
The case, however, was quite different in Infinity War. In the film, Stark used the abilities of his nanotech-powered suit to become one of the most efficient superheroes to take on the Mad Titan. He was even able to make Thanos bleed, despite having a moon literally dropped on him. However, Thanos eventually gained the upper hand and stabbed Tony with a blade of his own making. Still, he left Stark alive, leaving his final fate up to the snap of his fingers. A rematch finally came in Avengers: Endgame, which ended with Tony Stark heroically sacrificing himself to defeat Thanos once and for all.
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