The Magic Tower’s Problem Child - Chapter 25

You can see Oscar’s experience through these battles. He acts like he’s fallen into his opponent’s trap to get them to drop their guards. He takes precautions before the battle even begins. Such as covering his whole body with mana. And he’s created techniques that only a wind mage can. Such as creating clean oxygen inside his body.
That means he can hold his breath as long as he has mana. That’s an amazing application of magic. He can swim as long as he wants.
If these guys knew they were fighting a pre-teen they’d kill themselves. Well they aren’t really. Oscar has lived decades as a tower master.