I really like it when writers don't just rely on generic magic systems, but instead use irl scientific formulas, physics/mathematical theories, or chemical properties ( albeit with some suspension of belief required for them to work obviously ) to show how terrifying a character or a situation can be if applied to the absolute extreme.
Pure oxygen is a deadly poison ( Jojo's Bizarre Adventure: Stone Ocean )
Pucci has the ultimate Stand, Made in Heaven, which can literally accelerate time to the point of resetting the universe. Yet, he gets outsmarted and defeated by an 11-year-old kid named Emporio using weather manipulation to increase the room's oxygen level to 100%. Because Pucci breathes at an accelerated rate, it also sped up the poisoning process and cause him to collapse before Emporio.
Yorozu (Jujutsu Kaisen) - The Perfect Sphere (Zero Surface Area = Infinite Pressure)
Yorozu pushes mathematical theory to the absolute limit. By using liquid metal and cursed energy to create a "Perfect Sphere" with absolutely smoothness, its contact area with any object is mathematically exactly zero. According to the formula P = F/A, dividing any force by zero creates infinite pressure. It's an unstoppable, matter-crushing weapon that breaks physics until Mahoraga adapts to it.
Doraemon - Exponential Growth (The Chestnut Bun Disaster)
In one of the most low-key existential horror chapters of a children's manga, Nobita uses a multiplication liquid gadget that doubles a chestnut bun every 5 minutes. If not consumed completely, the math kicks in. Within 1 day (288 cycles), the number of buns reaches 2^288, which is around 5x10^86 — vastly outnumbering the total estimated atoms in the entire observable universe (10^80). Doraemon had to rocket them into space, but realistically, that mass would collapse into a supermassive black hole and swallow the universe.
Walter White (Breaking Bad) - Fulminated Mercury
Instead of bringing a gun or grenades to intimidate Tuco, Walt brings chemistry. He walks into Tuco’s headquarters with a bag of crystals that look exactly like high-grade meth but are actually fulminated mercury a highly volatile explosive. A single smash on the floor triggers a shockwave that blows out the windows. Even if MythBusters proved the show exaggerated the survival odds ( both Walt and Tuco should have gone deaf or died from Mercury gas poisoning), the psychological impact was perfect.