While the boss does despawn and doesn’t allow you to [i]immediately[/i] resummon for another shot, he isn’t gone “forever”; you can make him reappear by [b]resetting[/b] your Mythic World Tier using the Skull on the Kepler map and running back to his hidey-hole from where you spawn in at the Caldera. As such you can attempt the fight as many times at it takes to win, just with an annoying run back from the Altar each time you die. (΄◉◞౪◟◉｀)