From Wiktionary, the free dictionary
From boss + fight.
bossfight (plural bossfights)
- (video games) A fight against a boss.
2017 October 2, Heather Alexandra, “Cuphead Player Beats Bosses Without Taking A Single Hit”, in Kotaku[1]:Fireb0rn takes it a step further, spamming special attacks to dominate bossfights even if they get hit from time to time.
2021 November 26, John Walker, “14 Fascinating Unknown Indie Games To Play Straight Away”, in Kotaku[2]:Out already, free via Itch, The One Who Struggles is a game consisting entirely of one multi-phase bossfight.