Summaries for dummies
A kid with anger issues punches monsters really hard and somehow that's the whole plot.
One Punch Man (Saitama) — also accept Attack on Titan loosely, but OPM is the answer
A teenager goes through puberty twice in two different forms and just kills everyone
Attack on Titan
Student writes in his diary and becomes the world's most delusional criminal justice system.
Death Note (Light Yagami)
A boy breaks both his arms meets god and his entire country goes to war.
Fullmetal Alchemist: Brotherhood