Where in the side story, a guy got summoned into a game world.
Realized unlike in the game he's fucking terrified of dying (also the leveling requirement in the game would be absurd to do in real life)
But then discovered the power of friendship (i.e. party up to take on bigger foes)
However, even as he's gaining fame and being praised as a hero, his mind always worry.
That if this is that game's world, will that 'joke' of an event boss appear.
It appeared, and although the flow of the fight went differently than the game, everything that made that fight such a 'joke' was very much the same.
The overpowering strength that can kill you in one hit, through shield.
The insane speed that can run past the tanks to the supports.
Magic that's strong enough to take down multiple people from afar (also too fast to dodge)
In the game, one of the healers complained "You guys! Stop dying so much!" and his guild decided "you know what, this fight has 1 hour time limit, let's just figure out a way to survive that long first"
So they did, and at the end of that they were met with "I've had enough, I'm going home" (i.e. the boss had enough fun fighting) and that count as a clear, much to all their surprises.
In reality? It took a 'Goddess' to intervene, and 3 days of destruction later the boss said that exact same line with the Goddess, stripped naked after taking a strike that seem to tear the sky apart, prostrating in defeat.
Said event boss is the MC of the story proper.
To paraphrase the author: "A minor Goddess who's roleplaying as a Demon Queen"