Final boss
Etymology
From video-game design, where the final boss is the last and toughest enemy of a game, more powerful than the mid-level bosses before it. The phrase spread as a metaphor for any culminating challenge, and as a wry label for a person who seems to embody a category’s extreme form.
noun · /FY-nuhl BAWS/
The ultimate, hardest obstacle or adversary — the last and most formidable challenge in a sequence.
What it names
It borrows video-game structure to frame real difficulty as a climax: the culminating opponent you face only after clearing everything else. Calling something the final boss casts a whole ordeal as a game with levels, and this as the last one — the test that everything before was preparation for.
Example
Passing the courses was fine; the dissertation defense was the final boss.
Related idea
More than an ordinary obstacle: a final boss is specifically the last and hardest one, the climax of a whole progression.