Tsuki ga Michibiku Isekai Douchuu - Vol. 4 Ch. 26

[Pallington]

lol, she says she and Mio would lose, and yet he lost against Mio in their first fight, fainting like a little b*tch. So tired of such stupidly forced situations/plot holes.

1: there's context missing in the manga, he's gotten more proficient at spells and nuking since then. I mean, it's been 2 weeks, he has infinite fucking mana (more or less) and he was only OK at spell usage then.
2: he was basically taken by surprise, immediately put and locked into close range, mostly unequipped (no bow, no armor, only a knife), didn't have a full grasp of his abilities, didn't want to accidentally friendly fire the surroundings, and had to keep the spider busy. There's a mountain of conditions against him. If even a few of these conditions didn't apply (especially no-friendly-fire and not being immediately locked into close range, which he can enforce now with mist gates) then yeah, they'd be sitting ducks as he bombards them from outside their range.