Let me bring another example of a character that is extremely powerful but may not be a Marty Stu. One-Punch Man. Because he literally defeats numerous monsters with only one punch. No monsters can beat him. At all. I swear, by [deity]'s name.

But One-Punch Man still has problems. In his world, he's considered a madman on speedos as he's not licensed into the central superhero organisation yet. After the examination, his 'student' is far higher rank than he is, and far more popular. And his cute, weakling face does not help project power to people. So no one, NO ONE, believes his achievements or power even when they see it in front of their eyes. And when people see it? They thought he's a scam, tagging along S-class heroes to raise his rank. Or using them so he gets the final 'experince' points after they beat the monster almost to a pulp.

The guy's challenges include acceptance into a society that doesn't believe of his might, his boredom over the lack of enemies equal to his strength, and his baldness. He madehen judge for yourself whether he's a Marty-Stu or not.