I Just Graduated High School, and the System Wants Me to Slay a God?

[Farmer’s Three Punches + no saintly do-gooder + sneaky schemer + lightning-fast leveling + one heroine + comedy] Lin Fan transmigrates into a high-martial world of swords and magic. Just after graduating high school at sixteen, the Godslayer System arrives. The bad news: the System showed up a thousand years too early, and keeps telling him to go kill the Supreme Demon Empress. The good news: the Demon Empress is only eighteen, pure and innocent, and a little absent-minded. She should be easy to handle.