Ziggurat

To reach the end of the game faster, play on Easy difficulty. You can toggle difficulty at the character select screen when you start a new game.

The final showdown takes place against Examiner Cepheus. You'll find him in the boss room of the fifth floor. Like other boss rooms, you need the Portal Key of that floor in order for the boss to appear in his room.

Just die and the Achievement will unlock. I got this one naturally because the fight is pretty hard. You might want to try beating him first and seeing how the fight goes.