How Do You Turn Yourself into a BOSS in a Stand-Alone Game?

A player who is a heavy plot fan stayed up late to evaluate the single-player "Ashes Oath" in the Western Fantasy Wasteland. After swiping to turn on "Deep Immersion", he transformed into a dead Ember Knight. He was surrounded by the silver-haired saint Celia who only had 99% favorability but could not conquer the game. In order to rewrite her inevitable fate, he went back crazy, stole dragon bones, grabbed fire, and bombed the royal capital, almost distorting the world line. However, when he restarted for the Nth time, he was shocked to realize that he outside the screen had been copied by the system as a red-eyed BOSS, and he was walking towards Celia with a sword - every time he read the file, "player" was rewritten as "destroyer". In order to end the infinite loop, he must personally defeat the final BOSS, which is composed of his own data, and create the only true ending for Celia and himself at the crash save point.