You may now reach the elevator for the last time, and use the newly unlocked switch (the above screenshot). Wait now for the elevator to return to the first level of the dungeon.
Make sure that the green beam is directed towards the northern door (the above screenshot), and go there.
[TIP: since there is a battle with another boss drawing near, considered the idea of returning to the city for supplies, or explore the area of the City of the Dead once again.]
Keep going north. The place that you need to reach is the shown in the above screenshot arena. Entering the location will initiate a boss battle.