In this level, you need to transverse the whole thing on floating railways, trains, and rubbles from the stations in the Ikebukuro area. It's also a big maze, though unlike post #3762795, you can explore it just fine without a guide by using the minimap.
The boss (a "laser cannon dragon". You also need to destroy the laser cannon obstacles besides defeating the lesser dragons scattered around the area) is located in that "celestial globe" in the background, going there by using the railways conveniently placed leading to it.
(Yes, you're indeed running around in the dungeon several hundred metres above the ground. Scary, I know.)