My Hero Academia introduces a shonen hero on a mission, Izuku Midoriya. He's here to save the day, and Izuku understands that if he wants to become the next symbol of peace, he'll have to put in some serious legwork -- and he's been training non-stop since his first fateful meeting with All Might himself. He'll take on any challenger to achieve his dream.
Fullmetal Alchemist, on the other hand, features Roy Mustang as a supporting character, a borderline antihero who plays off well with Edward Elric (the actual protagonist). If Roy Mustang faced Izuku in battle, only one of these shonen heroes would remain standing.
Izuku Midoriya's Skills & Powers
Izuku Midoriya was born without a Quirk, so he compensated by using his sharp mind to study and assess the Quirks of other people instead. For years, Izuku kept a notebook to log the Quirks and fighting styles of other heroes and villains, and he's still doing this at U.A., where he is a student. Izuku is easily able to analyze the inner workings of any Quirk or combat technique and determine its strengths and weaknesses, allowing him to plan around the enemy's strategy and take them by surprise.
Then, Izuku received the unique One For All Quirk from All Might himself, being worthy of succeeding him. This is a power-charging Quirk capable of explosive action, though it puts a lot of strain on Izuku's body in the process. Izuku slowly learned to wield this power correctly, especially when he visited Gran Torino during his hero internship. Izuku can deliver devastating Smash attacks from either fist, and he can use Shoot Style to deliver mighty kicks when using One For All's power. He can easily use 5% of One For All's power in battle, and if pushed, he can use 1,000,000%, such as during his fight against the villain Muscular. What is more, Izuku can spread One For All's power across his entire body to reduce strain, and this Full Cowling technique allows him to move quickly and with great agility, much like Gran Torino himself (and with great precision, too). Sir Nighteye was impressed when he saw Full Cowling in action.
Roy Mustang's Powers & Skills
Colonel Roy Mustang was gifted with the unique art of flame alchemy, the inventi0n of Berthold Hawkeye (father of Riza Hawkeye). Roy wears two special cloth gloves that are designed to create sparks when he snaps his fingers, and he has a special transmutation circle on each glove to make this alchemy work. But there's more to it than just generating sparks.
Roy will actually use his alchemy to rearrange the oxygen atoms in the air to surround his enemy, and once they're all in position, he can set them ablaze with the sparks from his finger snaps. As a result, Roy's explosions and fire attacks are rather precise. There was a time when a handful of Roy's allies were surrounded by a crowd of enemies, and Roy carefully threaded his alchemy through the crowd so he only hit specific targets. The explosions are also incredibly powerful, being able to incinerate many people at once and blast apart walls or vehicles. When Roy repeatedly blasted Lust the homunculus with his alchemy, he wore down her Philosopher's Stone and killed her for good.
If Roy needs to perform a precise attack, he'll use his left hand instead. Roy's left hand can boil away the water in someone's eyes or tongue, as he did against Lust's teammate, Envy, and this can quickly incapacitate a foe while avoiding killing them. No matter which hand he uses, Roy won't tire too quickly; he can keep up these fire attacks all day long, only needing the effort to raise his arms and snap his fingers.
Izuku Midoriya Vs. Roy Mustang - Who Would Win?
In a My Hero Academia Vs. Fullmetal Alchemist showdown, Izuku would leap into action at once when the fighting starts, activating Full Cowling to maximize his mobility. Just in time, too; Roy Mustang only needs to raise his arm and snap his right hand's fingers to begin his fiery assault, and Izuku would only narrowly evade Roy's opening attacks.
During this time, Izuku would keep his eyes on Roy to assess that alchemist's fighting style, at which point it would become clear to Izuku that Roy is a ranged fighter who relies on firepower to win, much like Katsuki Bakugo (minus the bad attitude). Izuku can keep it up, though, leaping around the battlefield and keeping Roy on his toes. Roy's alchemy is a little too slow to keep up with a quick target like this, and Izuku is quicker than any of Roy's previous enemies or allies, such as the agile Edward Elric.
Izuku could then charge in for a counterattack, making use of the smoke from Roy's attacks as an actual smokescreen. Roy may try to fire off another explosion at close range or even use his left hand's ability, but it would be too late by then. Izuku would have already been charging up a 5% smash, and he'd unleash it at once. The sheer force of a Smash attack could blow all the air away and disrupt Roy's alchemy, protecting Izuku from having his eyes or tongue boiled away.
By then, Roy would have run out of options, and be blasted away by the Smash attack. Even if he gets up from that, Izuku would close in on him and finish him with Shoot Style kicks. Roy would lose this anime battle.
