The MC exists in a world where magic duels are the peak of culture and he writes an anonymous opinion column discussing the odds of each fight. His reports are well-respected, but he's known for always betting against the king of the arena, a summoner who calls down angels, even though the summoner is undefeated. In reality, the MC always wanted to be an arena fighter, but could never hack it, becoming an analyst for the police instead, learning about all kinds of magic he could never manage to use.
He gets his chance when one of the arch-mages, a notorious demon-blooded woman, discovers his column and tracks him down, offering to make him her apprentice. He accepts and starts training under her, getting involved in various undercity occurrences.
Eventually the reason he could never master any magic is reveled to be that he was too afraid that he'd pick the wrong path, so he let his potential waste away. His master makes him swear to pick one school of magic, and only ever practice that school to achieve his ambition, so, naturally, he choses the strongest magic: Angelic Summoning. But he's not beloved by the divine, nor does he have the mana to compel angels to obey him or even hear his calls. So he has to travel to the lowest point in the city and perform a ritual to attract the angels, opening the gates of hell a smidge, then permanently sacrifice a part of his spirit to make chains to enslave one of the divine beings to his will.

The story and MC would start out as more mundane, but as things pick up he'd grow a little crazier as he enjoyed finally pursuing his path in earnest and finding more insane uses for angels, reflecting the King Summoner's own strategies with more pragmatic tactics like instead of cherubs running disruption he'd have them explode in his opponent's face.