I'm not sure specifically about a story named 'everyone vs mimi imfurst'. It could be something very specific within a certain community or context that I'm not familiar with.
Guts is purely fictional. It's a concept or character that exists within the realm of imagination and creative storytelling rather than being based on reality.
Well, the Naruto vs Pain fight in the original series is epic. Fanfiction allows fans to explore different scenarios and what - ifs. Maybe they want to see different character developments during that intense battle.
Perhaps in this fanfic, Griffith has turned into something truly evil and a threat to the world Guts cares about. Guts, being the warrior he is, might feel it's his duty to put an end to Griffith to save others. Also, Guts could be driven by his own sense of justice. If Griffith has wronged innocent people, Guts may decide that killing him is the only way to set things right.