As you finished Quest 5, the door beside the Durance of Hate will open. Find the final Boss Mephisto at the third floor of Act 3. Before reaching the third floor, here I recommend walking counterclockwise along the edge of the map, and finding a waypoint when you are in the second floor.

The damage for Mephisto are extremely high that will cause death of your character, so waypoint can help shorten distance and time. Monsters in the former two floors are already strengthened. You'd…