In chapter 11 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 transfer student Ritsu, 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 Itona Horibe, 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.
For the final touch, the Chairman shows 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.
It's later revealed that they possess Ninja sneak walk techniques.
Karma in chapter 64 and 65. This kid is unstoppable.
Koro-Sensei and Class-E defeating Gastro in chapter 68. With a badass tactic of confusion and bringing the stage lights down on him.
Little Nagisa making a clown out of Takaoka again in chapter 72, with a technique derived from sumo wrestling no less. Then he (inadvertently?) inflicts mundane Mind Rape on the guy by smiling as he knocks him out.
Terasaka should get some credit, too. Not only does he ignores the effects of a deadly virus for hours while others were knocked out after ten minutes, his words of encouragement allow Nagisa to snap out of his rage-induced breakdown.
Chapter 79. All of Chapter 79. Takebayashi just became a Memetic Badass.
To expand upon that, Chairman Asano gave Takebayashi the chance to get a HUGE boost in power and respect by giving a speech lying about Class E, deriding them, and saying that he wanted to create a separate class that would "reform" them. He seems to consider it but, instead, the following happened when he was on stage to give his speech to the whole school:
Takebayashi:In Class E, in which I stayed...is a gathering of weak people. Because they lack what we call Academic Strength, they receive discriminatory treatment from everyone in the Main Building. But, for me, Class E is almost as comfortable to be in as a Maid Cafe...I was lying. Because I wanted to be strong. Because I wanted to be accepted. But, even though I was the most useless among Class E, and betrayed them on top of that, my classmates still came to see how I was doing, many times. Sensei, even for a student like me who had no knack for studying - he tried every method possible and schemed to teach me. I, who was unaccepted by my family and all of you, was treated as an equal by Class E. All of you, who are aiming to become distinctly strong and accepted by society...I believe you are right, and I respect you. But, for a little while longer, I'm fine with being weak. While bearing with weakness, and enjoying weakness, I will return to my life of aiming for the necks of the strong. [he is scolded by Gakushuu, and pulls out the plaque from the chairman's office] I swiped this from the Chairman's room. It is like a shield which commends a private school's best manager...The Chairman is truly a strong person. All his actions are rational. [he smashes the plaque to pieces, using a knife] According to Asano-kun, in the past, another student did this same thing, or something like that. Thinking rationally, based on precedent, I'd say it's off to Class E for me as well. [he starts to leave the stage]
Gakushuu:Hold it. [he claps a hand on Takebayashi's shoulder] There really is no saving you, is there? And here we went through all this trouble to give you the chance to become strong.
Takebayashi:Strong? It looked like a group of people who were just scared. You and everyone else.
The reactions of Gakushuu and his father made this all the more amazing. Gakushuu almost completely lost it, becoming angrier than ever, when Takebayashi made it clear that he saw right through him. The Chairman looked livid at Takebayashi's defiance, and no longer seemed to be in control of the situation at all. One of his "pawns" had flouted the system, in front of the entire school. Who knows what long-term effect it will have?
And, as if all of that wasn't awesome enough, it turns out that Takebayashi's going to memorize all of the info the class needs to know about explosives, so that he can teach the rest of the class how to use them against Koro-sensei.Goddamn, Takebayashi.
Chapter 80 has Kayano's gigantic freaking pudding. With explosives inside the bottom.
Chapter 81: Is Karasuma-sensei really human?
Class E manages to outsmart Karasuma and Koro-sensei by leaving one squad in the pool so that it can't be captured by Koro-sensei, and another one to challenge Karasuma-sensei and lead him far away from the pool.
Chapter 84: Koro-sensei defeats Itona a third time. This time it's easier, and he even pulls off a new move he recently learned.