We get it: Deku isn’t trying to “trick” or “underestimate” Kacchan. He simply has to believe he can surpass someone as amazing as Kacchan if he’s ever going to develop into reliable hero. So while the trial is supposed to be about heroes and villains, Kacchan makes it into a duel of nemeses, and Deku has to choice but to play along, while trusting Ochako to handle the bomb retrieval.
Deku’s notebook knowledge serves him well against an unfocused and increasingly angry Kacchan, but as he gets worn down from all the dodging, and Kacchan gets angrier stalking through the halls, remembering all the times Deku proved himself useless when they were little kids, Kacchan devises more and more subtle yet devastating attacks.
Mind you, that’s after he blasts a hole in his own “villain stronghold”, leading All Might to warn him if he causes that much destruction again, he’ll forfeit the match.
But he and Deku both know Kacchan doesn’t give a shit about the outcome of the match. He wants Deku to know his place. And All Might knows Deku won’t make any progress getting through to Kacchan if he suspends the match.
The duel culminates in the two driving their fists at each other, Kacchan with his explosive power and Deku with All For One—but Deku isn’t going for Kacchan, he’s going for the ceiling.
By blasting numerous holes in the building, he creates a diversion, as well as ammo, for Ochako to wield her antigrav powers and make contact with the bomb, flummoxing Iida, who had tried so hard to play the role of mustache-twirling villain.
As time runs out and the Hero team wins, Kacchan is still playing the same refrain: “Don’t underestimate me. I’m better than you.”
Well, talent and strength, especially in the quantities he possesses, certainly are great to have. But that alone doesn’t make a hero. Kacchan seemed constantly driven by hatred for that which he always thought was weaker than him, but day by day is being proven wrong, making him question his own worth deep inside.
Add to that nitroglycerin palm sweat (how the hell did he not accidentally burn his house down nine thousand times as a kid?), and you have a volatile combination. But when Deku tells Kacchan can’t use his quirk lest it destroy his body, and Kacchan sees the damage to prove it, his scowl of contempt softens into something resembling pity, maybe even understanding and regret for what he’s put Deku through.
Because I feel like a lot of his anger has to do with the fact that Deku never once deserved the shitty treatment Kacchan dumped on him. Quite the opposite, whenever Kacchan even looked like he was in trouble, Deku and only Deku rushed out to help him. Just as Deku needs to strengthen his body and master his quirk to have a future as a hero, Kacchan will have to resolve his various emotional issues.