One-Punch Man: I Began as a Martial Artist and Brutalized Saitama!

[One-Punch Man fan fiction] [System] [Leveling] After transmigrating into the world of One-Punch Man and unlocking the Lava Fighter System, can I finally leave behind my ordinary past life and rise to the top like a hero?! Just as Li Xin was congratulating himself, the System issued a main quest so daunting it made him want to back away! [Surpass Saitama within three months, or be physically erased!] Am I really supposed to fight Saitama in nothing but my underwear? Is this for real?! Saitama is the strongest, according to the original story's very foundations. How could I possibly beat him?! As Li Xin grows stronger, Genos hunts him down while Fubuki calls him “sir”! Who knows how it will all turn out, but at least his life has finally become interesting.