Part 1 (short fuse)
During duel class, bakugou is antsy.
Bakugou: come on, all you extras are getting stomped, if I knew you all lacked skill I would have held back.
Izuku: kacchan, stop, or I'll duel you myself, and I'll use my HERO deck.
Now Izuku has a slight reputation in his class for his HERO deck, being called the unending king, namely because his twin trap combo makes it so he cannot lose due to deck out, recycling up to six cards at a time.
With this deck even bakugou is pressured to end it before his deck gets into the single digits, which shows how urgent things are, due to bakugou's strongest monster is a rock type.
(explo-tech mass gunpowder, a 12 star Fusion monster requiring three explo-tech monsters with different names in order to be fusion summoned, when this card is fusion summoned it is treated as both machine type and rock type, it's attack is equal to the combined attack of its material plus 2000.)
Now they duel and bakugou gets out explo-tech mass gunpowder, and due to he destroyed all of izuku's monsters, he ends his turn.
Izuku: my turn, draw, (hand consists of kuriboh, dystopia the despondent, winged kuriboh, five star twilight, super polymerisation and call of the haunted) I summon winged kuriboh, I then activate my five star twilight, which thanks to one of the kuri siblings abilities, allows me to special summon kuribabylon, then I play my super polymerization.
Bakugou: what?! What the hell does that do.
Izuku: this card allows me to fuse one of my monsters, along with one of yours, to create a new one, and that monster is evil HERO dark Gaia, who's attack points is the combined number of our monsters attack points, dystopia had 5000 attack points, your mass gunpowder had 6000, that equates to 11000 attack points, battle phase, dark Gaia attack.
With that, Izuku wins amazing everyone and reminding them of how powerful Izuku is, much to bakugou's distaste, he lost and in the same way he beats extras.
How far behind was he, that was izuku's HERO deck, not his real one and even the he got stomped out, and that spell card, how come he didn't know about it, his explo-tech's would benefit from it, super polymerization, such a powerful card, the fact he could turn an opponent's monster against them is terrifying, and that dark Gaia just gained power from it like it was nothing, it was overkill even for bakugou's taste.
They only had 8000 life points, and bakugou never gained any, so 11000 is 3000 points of pure overkill.
Bakugou was fuming about that, but had to admit defeat because of what happened.
Back in class.
Extra 1: yo Izuku, can we see your HERO deck.
Izuku: sure, have a look.