In one of the recent chapters two of the normal students try to bully Nagisa. Nagisa simply laughs them off and sends them running back to their teachers.
In chapter 51 the elite of Class A try to intimidate them by wagering their lives on a test...it ended poorly for them.
Koro-Sensei going so fast to help each student individually so they can score at the top 50 for their next exams is pretty cool to see (and heartwarming)
Many of Koro-Sensei's actions are either this, CMOF, or CMOH.
Karma. Nuff said.
Class E's newest addition, a learning AI, packed to the circuit boards with guns, blowing off Korosensei's index finger by repeating her previous attack exactly, and hiding a bullet behind the one she knows he'll deflect.
New transfer student Horibe Itona, lopping Koro-sensei's arm off.
Koro-Sensei VS Itona in chapter 31.
Chapter 36, Class E manages to beat the Baseball Club. Despite the Chairman's helping them.
Chapter 38. Nagisa's killing intent is so strong and he moves so quietly and quickly, Karasuma has a vision of a viper, coiled around his neck and already poised to strike. Karasuma, The Stoic extraordinaire, panics and knocks Nagisa away with his full strength.
Then in Chapter 40, Karasuma chooses Nagisa to fight Takaoka, the new Sadist Teacher on the block, as a direct result of Chapter 38.
Then in chapter 41, the foreshadowing bears fruit. Nagisa is explicitly mentioned to have a great talent for assassination, because of his talent to control his bloodlust, to frighten others with bloodlust alone, and to be at ease when acting in front of an audience. He demonstrates this by taking down Takaoka with scornful ease, while looking like he was just walking to school. He even shocks Karasuma by far exceeding his expectations, which he amusingly misinterprets as Karasuma expecting him to use the sharp side of the blade, suggesting that he truly had no compunctions against using a real knife against a person.
Capped off in chapter 42, when Takaoka is raging at Nagisa for a rematch, promising to beat him to a pulp. Nagisa simply bows, thanks him for his hard work, and politely asks him to leave. Kid's got a spine of steel when he's in the mood.
And Karasuma pwned Takaoka◊ with one hit while not even looking at him.
The Chairman showing up, and firing Takaoka.
Class E defeating Itona. By way of a second day worn shirt covered in Anti-Sensei powder, and a giant cannonball into the pool.
The whole chapter 53. Tests have never being so epic! To summarize, we have the tests as giant armors which hided a lot of traps beneath and the two classes battled them with spells, hammers, Naginatas, and guns.
Before that, there was Chapter 14, where answering the questions on the midterm tests are regarded as something akin to slaying gigantic, carnivorous monsters.
Yuuma, Manami and Rio taking the number 1 spot in Social Studies, Science and English from Asano.
During the first year finals, Class E manages to completely trounce Class A's test scores and monopolize most of the top 50 spots in the school. They also pull one over on Koro-sensei by having a bunch of their weaker students all get the top scores in home economics. As part of the agreement they made with Koro-sensei, their Loophole Abuse allows them to get a huge handicap next time they fight him. They also manage to take Class A's special summer resort trip.
Regardless of its actual effectiveness, the sheer complexity and scope of Class 3E's plan in chapter 59 is pretty awesome and shows how much they know about their target, and how serious they are. The only reason they failed was because they were 0.5 seconds too slow in the delivery. If they'd been just slightly faster, even Koro-Sensei's ultimate defense would have been useless.
Koro-sensei sets up a plan that even the pros think is suicidal and impossible. Largely because it involves the students scaling a cliff. Just as Karasuma is ready to give up he looks up to find...the class is already halfway there. Class 3-E are now ninjas
Karma in chapter 64 and 65. This kid is unstoppable.