One-Punch Man basically just.. destroys everything with one punch. If I recall right, Flandre got locked up in the basement because she destroys things with ease.
She got locked up for having no control. She's slightly more powerful than Remilia, but nowhere near as powerful as some others. She just has absolutely no restraint on how she uses her power, and therefore was a dangerous loose cannon.
I will admit that I do not know Koishi that much because I haven't played Subterranean Animisim (And even if I did, I probably won't be skilled enough to reach the Extra boss stage), and had to look up the wiki.
Come to think of it, Touhou fairies just respawn after death. So she's functionally immortal. And she's strong enough offensively to give Marisa a hard time, with solid defense, as well. Tough enough to fight inside a nuclear reactor against a nuclear chicken, at least. If she weren't so stupid and easily amused, she might actually would be among the strongest.
Of course when... so many characters are immortal, it's really hard to say who's really strongest.
Both are based off of the same source material, but due to the different art styles and character portrayals they feel completely different and can be watched back to back.
Seeing how the girl in the center is Tamamo, the same girl in the above game screenshots. She's from Monster Girl Quest... have fun with that information
iirc, earlier in dungeon you save fox mode 2 tailed fox girl from being eaten by giant spider girl. now at end of dungeon, rather than just completely and utterly curb stomping you, 9 tailed fox girl decides to reward you with either fried tofu, best option, or quest item